A Sol-Gel/Solvothermal Synthetic Approach to Titania Nanoparticles for Raman Thermometry.

The accurate determination of the local temperature is one of the most important challenges in the field of nanotechnology and nanomedicine. For this purpose, different techniques and materials have been extensively studied in order to identify both the best-performing materials and the techniques with greatest sensitivity. In this study, the Raman technique was exploited for the determination of the local temperature as a non-contact technique and titania nanoparticles (NPs) were tested as nanothermometer Raman active material. Biocompatible titania NPs were synthesized following a combination of sol-gel and solvothermal green synthesis approaches, with the aim of obtaining pure anatase samples. In particular, the optimization of three different synthesis protocols allowed materials to be obtained with well-defined crystallite dimensions and good control over the final morphology and dispersibility. TiO2 powders were characterized by X-ray diffraction (XRD) analyses and room-temperature Raman measurements, to confirm that the synthesized samples were single-phase anatase titania, and using SEM measurements, which clearly showed the nanometric dimension of the NPs. Stokes and anti-Stokes Raman measurements were collected, with the excitation laser at 514.5 nm (CW Ar/Kr ion laser), in the temperature range of 293-323 K, a range of interest for biological applications. The power of the laser was carefully chosen in order to avoid possible heating due to the laser irradiation. The data support the possibility of evaluating the local temperature and show that TiO2 NPs possess high sensitivity and low uncertainty in the range of a few degrees as a Raman nanothermometer material.

Kinetically Stable Nonequilibrium Gold-Cobalt Alloy Nanoparticles with Magnetic and Plasmonic Properties Obtained by Laser Ablation in Liquid.

Nonequilibrium nanoalloys are metastable solids obtained at the nanoscale under nonequilibrium conditions that allow the study of kinetically frozen atoms and the discovery of new physical and chemical properties. However, the stabilization of metastable phases in the nanometric size regime is challenging and the synthetic route should be easy and sustainable, for the nonequilibrium nanoalloys to be practically available. Here we report on the one-step laser ablation synthesis in solution (LASiS) of nonequilibrium Au-Co alloy nanoparticles (NPs) and their characterization on ensembles and at the single nanoparticle level. The NPs are obtained as a polycrystalline solid solution stable in air and water, although surface cobalt atoms undergo oxidation to Co(II). Since gold is a renowned plasmonic material and metallic cobalt is ferromagnetic at room temperature, these properties are both found in the NPs. Besides, surface conjugation with thiolated molecules is possible and it was exploited to obtain colloidally stable solutions in water. Taking advantage of these features, an array of magnetic-plasmonic dots was obtained and used for surface-enhanced Raman scattering experiments. Overall, this study confirms that LASiS is an effective method for the formation of kinetically stable nonequilibrium nanoalloys and shows that Au-Co alloy NPs are appealing magnetically responsive plasmonic building blocks for several nanotechnological applications.

Hybrid Sol-Gel Surface-Enhanced Raman Sensor for Xylene Detection in Solution.

This paper reports on the fabrication and characterization of a plasmonic/sol-gel sensor for the detection of aromatic molecules. The sol-gel film was engineered using polysilsesquioxanes groups to capture the analyte, through π-π interaction, and to concentrate it close to the plasmonic surface, where Raman amplification occurs. Xylene was chosen as an analyte to test the sensor. It belongs to the general class of volatile organic compounds and can be found in water or in the atmosphere as pollutants released from a variety of processes; its detection with SERS is typically challenging, due to its low affinity toward metallic surfaces. The identification of xylene was verified in comparison with that of other aromatic molecules, such as benzene and toluene. Investigations were carried out on solutions of xylene in cyclohexane, using concentrations in the range from 0 to 800 mM, to evaluate the limit of detection (LOD) of about 40 mM.

Structure and vibrational properties of 1D molecular wires: from graphene to graphdiyne.

Graphyne- and graphdiyne-like model systems have attracted much attention from many structural, theoretical, and synthetic scientists because of their promising electronic, optical, and mechanical properties, which are crucially affected by the presence, abundance and distribution of triple bonds within the nanostructures. In this work, we performed the two-step bottom-up on-surface synthesis of graphyne- and graphdiyne-based molecular wires on the Au(111). We characterized their structural and chemical properties both in situ (UHV conditions) through STM and XPS and ex situ (in air) through Raman spectroscopy. By comparing the results with the well-known growth of poly(p-phenylene) wires (namely the narrowest armchair graphene nanoribbon), we were able to show how to discriminate different numbers of triple bonds within a molecule or a nanowire also containing phenyl rings. Even if the number of triple bonds can be effectively determined from the main features of STM images and confirmed by fitting the C1s peak in XPS spectra, we obtained the most relevant results from ex situ Raman spectroscopy, despite the sub-monolayer amount of molecular wires. The detailed analysis of Raman spectra, combined with density functional theory (DFT) simulations, allowed us to identify the main features related to the presence of isolated (graphyne-like systems) or at least two conjugated triple bonds (graphdiyne-like systems). Moreover, other spectral features can be exploited to understand if the chemical structure of graphyne- and graphdiyne-based nanostructures suffered unwanted reactions. As in the case of sub-monolayer graphene nanoribbons obtained by on-surface synthesis, we demonstrate that Raman spectroscopy can be used for a fast, highly sensitive and non-destructive determination of the properties, the quality and the stability of the graphyine- and graphdiyne-based nanostructures obtained by this highly promising approach.

Au-Ag Alloy Nanocorals with Optimal Broadband Absorption for Sunlight-Driven Thermoplasmonic Applications.

Noble metal nanoparticles are efficient converters of light into heat but typically cover a limited spectral range or have intense light scattering, resulting in unsuited for broadband thermoplasmonic applications and sunlight-driven heat generation. Here, Au-Ag alloy nanoparticles were deliberately molded with an irregular nanocoral (NC) shape to obtain broadband plasmon absorption from the visible to the near-infrared yet at a lower cost compared to pure Au nanostructures. The Au-Ag NCs are produced through a green and scalable methodology that relies on pulsed laser fragmentation in a liquid, without chemicals or capping molecules, leaving the particles surface free for conjugation with thiolated molecules and enabling full processability and easy inclusion in various matrixes. Numerical calculations showed that panchromism, i.e., the occurrence of a broadband absorption from the visible to the near-infrared region, is due to the special morphology of Au-Ag alloy NCs and consists of a purely absorptive behavior superior to monometallic Au or Ag NCs. The thermoplasmonic properties were assessed by multiwavelength light-to-heat conversion experiments and exploited for the realization of a cellulose-based solar-steam generation device with low-cost, simple design but competitive performances. Overall, here it is shown how laser light can be used to harvest solar light. Besides, the optimized broadband plasmon absorption, the green synthetic procedure, and the other set of positive features for thermoplasmonic applications of Au-Ag NCs will contribute to the development of environmentally friendly devices of practical utility in a sustainable world.

Contactless Temperature Sensing at the Microscale Based on Titanium Dioxide Raman Thermometry.

The determination of local temperature at the nanoscale is a key point to govern physical, chemical and biological processes, strongly influenced by temperature. Since a wide range of applications, from nanomedicine to nano- or micro-electronics, requires a precise determination of the local temperature, significant efforts have to be devoted to nanothermometry. The identification of efficient materials and the implementation of detection techniques are still a hot topic in nanothermometry. Many strategies have been already investigated and applied to real cases, but there is an urgent need to develop new protocols allowing for accurate and sensitive temperature determination. The focus of this work is the investigation of efficient optical thermometers, with potential applications in the biological field. Among the different optical techniques, Raman spectroscopy is currently emerging as a very interesting tool. Its main advantages rely on the possibility of carrying out non-destructive and non-contact measurements with high spatial resolution, reaching even the nanoscale. Temperature variations can be determined by following the changes in intensity, frequency position and width of one or more bands. Concerning the materials, Titanium dioxide has been chosen as Raman active material because of its intense cross-section and its biocompatibility, as already demonstrated in literature. Raman measurements have been performed on commercial anatase powder, with a crystallite dimension of hundreds of nm, using 488.0, 514.5, 568.2 and 647.1 nm excitation lines of the CW Ar+/Kr+ ion laser. The laser beam was focalized through a microscope on the sample, kept at defined temperature using a temperature controller, and the temperature was varied in the range of 283-323 K. The Stokes and anti-Stokes scattered light was analyzed through a triple monochromator and detected by a liquid nitrogen-cooled CCD camera. Raw data have been analyzed with Matlab, and Raman spectrum parameters-such as area, intensity, frequency position and width of the peak-have been calculated using a Lorentz fitting curve. Results obtained, calculating the anti-Stokes/Stokes area ratio, demonstrate that the Raman modes of anatase, in particular the Eg one at 143 cm-1, are excellent candidates for the local temperature detection in the visible range.

A Review on Surface-Enhanced Raman Scattering.

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

Surface plasmon resonance in gold nanoparticles: a review.

In the last two decades, plasmon resonance in gold nanoparticles (Au NPs) has been the subject of intense research efforts. Plasmon physics is intriguing and its precise modelling proved to be challenging. In fact, plasmons are highly responsive to a multitude of factors, either intrinsic to the Au NPs or from the environment, and recently the need emerged for the correction of standard electromagnetic approaches with quantum effects. Applications related to plasmon absorption and scattering in Au NPs are impressively numerous, ranging from sensing to photothermal effects to cell imaging. Also, plasmon-enhanced phenomena are highly interesting for multiple purposes, including, for instance, Raman spectroscopy of nearby analytes, catalysis, or sunlight energy conversion. In addition, plasmon excitation is involved in a series of advanced physical processes such as non-linear optics, optical trapping, magneto-plasmonics, and optical activity. Here, we provide the general overview of the field and the background for appropriate modelling of the physical phenomena. Then, we report on the current state of the art and most recent applications of plasmon resonance in Au NPs.

Laser generated gold nanocorals with broadband plasmon absorption for photothermal applications.

Gold nanoparticles with efficient plasmon absorption in the visible and near infrared (NIR) regions, biocompatibility and easy surface functionalization are of interest for photothermal applications. Herein we describe the synthesis and photothermal properties of gold "nanocorals" (AuNC) obtained by laser irradiation of Au nanospheres (AuNS) dispersed in liquid solution. AuNC are formed in two stages: by photofragmentation of AuNS, followed by spontaneous unidirectional assembly of gold nanocrystals. The whole procedure is performed without chemicals or templating compounds, hence the AuNC can be coated with thiolated molecules in one step. We show that AuNC coated with thiolated polymers are easily dispersed in an aqueous environment or in organic solvents and can be included in polymeric matrixes to yield a plasmonic nanocomposite. AuNC dispersions exhibit flat broadband plasmon absorption ranging from the visible to the NIR and unitary light-to-heat conversion. Besides, in vitro biocompatibility experiments assessed the absence of cytotoxic effects even at a dose as high as 100 µg mL(-1). These safe-by-designed AuNC are promising for use in various applications such as photothermal cancer therapy, light-triggered drug release, antimicrobial substrates, optical tomography, obscurant materials and optical coatings.

SERS Properties of Gold Nanorods at Resonance with Molecular, Transverse, and Longitudinal Plasmon Excitations.

The amplification of Raman signals of the heteroaromatic cation 1-(N-methylpyrid-4-yl)-2-(N-methylpyrrol-2-yl)ethylene (PEP+)) bound to Au nanorods (NRs) was investigated at different excitation wavelengths to study the effect of the laser resonance with the absorption band of the PEP+ moiety and with the two plasmon oscillation modes of the NR. Two different PEP+ derivatives, differing in the length of the alkyl chain bearing the anchoring group, were used as target molecules. Raman spectra obtained exciting at 514 or at 785 nm (i.e., exciting the transverse or the longitudinal plasmon band) present a higher intensity than that at 488 nm suggesting a higher Raman amplification when the laser excitation wavelength is resonant with one of the two plasmon modes. Moreover, considering results of Discrete Dipole Approximation (DDA) calculations of the local field generated at the NR surface when either the transverse or the longitudinal plasmon modes are excited, we deduced that the resonance condition of the 514-nm laser excitation with the absorption band of the dye strongly contributes to the amplification of the Raman signal.

Photoinduced electron-transfer in perylenediimide triphenylamine-based dendrimers: single photon timing and femtosecond transient absorption spectroscopy.

The excited state dynamics of two generations perylenediimide chromophores substituted in the bay area with dendritic branches bearing triphenylamine units as well as those of the respective reference compounds are investigated. Using single photon timing and multi-pulse femtosecond transient absorption experiments a direct proof of a reversible charge transfer occurring from the peripheral triphenylamine to the electron acceptor perylenediimide core is revealed. Femtosecond pump-dump-probe experiments provide evidence for the ground state dynamics by populating excited vibronic levels. It is found by the means of both techniques that the rotational isomerization of the dendritic branches occurs on a time scale that ranges up to 1 ns. This time scale of the isomerization depends on the size of the dendritic arms and is similar both in the ground and excited state.

Photophysical study of photoinduced electron transfer in a bis-thiophene substituted peryleneimide.

Based on femtosecond time-resolved spectroscopy and single photon timing experiments, intramolecular photoinduced charge transfer has been investigated in two systems containing a peryleneimide chromophore (P) and thiophene (T) groups. The first compound bearing a single thiophene ring (PT1) is used as model and shows a behavior similar to P, studied previously, while in the compound with two thiophene rings attached (PT2) electron transfer from the thiophene donor to the peryleneimide acceptor is observed in benzonitrile. Femtosecond fluorescence upconversion and femtosecond transient absorption experiments in benzonitrile indicate that this ion-pair state formation occurs in 19 ps. This ion-pair state then decays with two time constants of 1400 and 820 ps, probably corresponding to different conformations of the thiophene rings.