Paper-based plasmonic substrates as surface-enhanced Raman scattering spectroscopy platforms for cell culture applications.

The engineering of advanced materials capable of mimicking the cellular micro-environment while providing cells with physicochemical cues is central for cell culture applications. In this regard, paper meets key requirements in terms of biocompatibility, hydrophilicity, porosity, mechanical strength, ease of physicochemical modifications, cost, and ease of large-scale production, to be used as a scaffold material for biomedical applications. Most notably, paper has demonstrated the potential to become an attractive alternative to conventional biomaterials for creating two-dimensional (2D) and three-dimensional (3D) biomimetic cell culture models that mimic the features of in vivo tissue environments for improving our understanding of cell behavior (e.g. growth, cell migration, proliferation, differentiation and tumor metastasis) in their natural state. On the other hand, integration of plasmonic nanomaterials (e.g. gold nanoparticles) within the fibrous structure of paper opens the possibility to generate multifunctional scaffolds equipped with biosensing tools for monitoring different cell cues through physicochemical signals. Among different plasmonic based detection techniques, surface-enhanced Raman scattering (SERS) spectroscopy emerged as a highly specific and sensitive optical tool for its extraordinary sensitivity and the ability for multidimensional and accurate molecular identification. Thus, paper-based plasmonic substrates in combination with SERS optical detection represent a powerful future platform for monitoring cell cues during cell culture processes. To this end, in this review, we will describe the different methods for fabricating hybrid paper-plasmonic nanoparticle substrates and their use in combination with SERS spectroscopy for biosensing and, more specifically, in cell culture applications.

Colloidal synthesis of silicon nanoparticles in molten salts.

Silicon nanoparticles are unique materials with applications in a variety of fields, from electronics to catalysis and biomedical uses. Despite technological advancements in nanofabrication, the development of a simple and inexpensive route for the synthesis of homogeneous silicon nanoparticles remains highly challenging. Herein, we describe a new, simple and inexpensive colloidal synthetic method for the preparation, under normal pressure and mild temperature conditions, of relatively homogeneous spherical silicon nanoparticles of either ca. 4 or 6 nm diameter. The key features of this method are the selection of a eutectic salt mixture as a solvent, the identification of appropriate silicon alkoxide precursors, and the unconventional use of alkali earth metals as shape-controlling agents.

Correction: A study of the depth and size of concave cube Au nanoparticles as highly sensitive SERS probes.

Correction for 'A study of the depth and size of concave cube Au nanoparticles as highly sensitive SERS probes' by J. M. Romo-Herrera et al., Nanoscale, 2016, 8, 7326-7333.

Robust raspberry-like metallo-dielectric nanoclusters of critical sizes as SERS substrates.

Raspberry-like nano-objects made of large plasmonic satellites (>10 nm) covering a central dielectric particle have many potential applications as photonic materials, superlenses and (bio-) sensors, but their synthesis remains challenging. Herein, we show how to build stable and robust raspberry-like nano-systems with close-packed satellites, by combining monodisperse silica particles (80 or 100 nm diameter) and oppositely charged noble metal nanoparticles (Au or Ag) with well-defined sizes (10-50 nm). The spectral characteristics of their associated plasmonic resonances (wavelength, linewidth, extinction cross-section) and the electromagnetic coupling between satellites were observed using the spatial modulation spectroscopy technique and interpreted through a numerical model. The composite nano-objects exhibit numerous hot spots at satellite junctions, resulting in excellent surface-enhanced Raman scattering (SERS) performance. The SERS efficiency of the raspberry-like clusters is highly dependent on their structure.

Silver colloids as plasmonic substrates for direct label-free surface-enhanced Raman scattering analysis of DNA.

Ultrasensitive direct SERS analysis offers a powerful analytical tool for the structural characterization and classification of nucleic acids. However, acquisition of reliable spectral fingerprints of such complex biomolecules poses important challenges. In recent years, many efforts have been devoted to overcome these limitations, mainly implementing silver colloids as plasmonic substrates. However, a reliable cross-comparison of results reported in the recent literature is extremely hard to achieve, mostly due to the broad set of different surface properties of the plasmonic nanoparticles. Herein, we perform a thorough investigation of the role played by the metal/liquid interface composition of silver colloids in the direct label-free SERS analysis of DNA. Target molecules of increasing complexity, from short homopolymeric strands to long genomic duplexes, were used as probes. We demonstrate how apparently subtle changes in the colloidal surface chemistry can dramatically modify the affinity and the final SERS spectral profile of DNA. This has significant implications for the future design of new analytical strategies for the detection of DNA using SERS without labels.

A study of the depth and size of concave cube Au nanoparticles as highly sensitive SERS probes.

High and uniform near fields are localized at the eight similar sharp corners of cubic gold nanoparticles. Moreover, by introducing concavity in the particle lateral planes, such field intensities can be further increased and tuned in the near infrared region without altering the overall size of the nanoparticles. Herein, we perform a thorough investigation of the morphological, crystallographic and plasmonic properties of concave gold nanocubes (GNCs) in the sub-70 nm size range, for their potential application as highly efficient SERS substrates in size-limiting cases. Theoretical calculations indicate that the highest increment of the near-field is located at the eight sharp tips and, interestingly, a medium near-field increment is also activated over the volume next to the concave surface. Remarkably, the plasmonic response of the concave cubic morphology showed great sensitivity to the concavity degree. Experimental SERS analysis nicely matches the outcome of the theoretical model, confirming that medium-sized concave GNCs (35 nm side length) possess the highest SERS activity upon excitation with a 633 nm laser, whereas larger 61 nm side concave GNCs dominate the optical response at 785 nm. Due to their size-intensity trade off, we envision that such small concave gold nanocubes can provide a highly active and efficient SERS platform for size-limiting applications, especially when near infrared excitations are required.

Silicon nanoparticles as Raman scattering enhancers.

In this communication we demonstrate the large amplification values of the Raman signal of organic molecules attached to silicon nanoparticles (SiNPs). Light induced Mie resonances of high refractive index particles generate strong evanescent electromagnetic (EM) fields, thus boosting the Raman signal of species attached to the nanoparticles. The interest of this process is justified by the wide range of experimental configurations that can be implemented including photonic crystals, the sharp spectral resonances easily tuneable with the particle size, the biocompatibility and biodegradability of silicon, and the possibility of direct analysis of molecules that do not contain functional groups with high affinity for gold and silver. Additionally, silicon nanoparticles present stronger field enhancement due to Mie resonances at larger sizes than gold.

Retention of cobalt on a humin derived from brown coal.

In this work, the retention of cobalt on a humin (HU) derived from a brown coal is studied. Through a systematic and coordinated investigation of the behavior of the metal ions in solution (speciation diagrams as a function of pH) and their adsorption and precipitation processes with reactive functional groups of the solid (sorption isotherms), the interactions of different Co(II) species with HU are probed. To further confirm the nature of these interactions, the complementary spectroscopic techniques of FTIR, Raman microspectroscopy, UV-visible absorption and XRD are employed. Molecular modeling techniques are used to gain information about the stability of different Co(II) species as a function of pH, as well as the stability of Co(II) species complexed with benzoic acid, a common surface component of humic substances. It is found that the selectivity that humin has for different Co(II) species, as well as the amount of Co(II) that it can retain, are both highly dependent on pH. Through Raman microspectroscopy measurements, the presence and location of Co(OH)(2) precipitates on the surface of HU is confirmed.

Theoretical study on fulvic acid structure, conformation and aggregation. A molecular modelling approach.

The ubiquitous presence of humic substances (HS), combined with their ability to provide multiple sites for chemical reaction, makes them relevant to numerous biogeochemical processes such as mineral weathering, nutrient bioavailability, and contaminant transport. The reactivity of HS depends on their functional group chemistry and microstructure, which are in turn influenced by the composition of the surrounding media. In order to help towards an understanding of structure conformations and aggregation process of HS in soils and waters and to get a better knowledge of these kinds of materials, a fulvic acid (FA) has been modelled as a function of its ionic state under different conditions. Our proposed theoretical model based on the Temple-Northeastern-Birmingham (TNB) monomer fits well with experimental observations on the solubility (dipolar moment) and electronic and vibrational spectra of FAs. The presence of water molecules has a great stabilization effect on the electrostatic energy; this effect is greater as ionized rate increases. In vacuum, the non-ionized aggregated species are more stable than monomers because of the increase in their interaction due to H-bonding and non-bonding forces. When the molecules are ionized, no aggregation process takes place. In solution, the FA concentration is a critical factor for the aggregation. The system containing two FA molecules probably did not form aggregates because its equivalent concentration was too low. When the concentration was increased, the system gave rise to the formation of aggregates. The ionic state is another critical factor in the aggregation process. The ionized FA has a higher electric negative charge, which increases the energetic barriers and inhibits the approximation of FA caused by the Brownian movement.

Surface-enhanced Raman scattering on colloidal nanostructures.

Surface-enhanced Raman scattering combines extremely high sensitivity, due to enhanced Raman cross-sections comparable or even better than fluorescence, with the observation of vibrational spectra of adsorbed species, providing one of the most incisive analytical methods for chemical and biochemical detection and analysis. SERS spectra are observed from a molecule-nanostructure enhancing system. This symbiosis molecule-nanostructure is a fertile ground for theoretical developments and a realm of applications from single molecule detection to biomedical diagnostic and techniques for nanostructure characterization.

Effect of pH on the aggregation of a gray humic acid in colloidal and solid states.

Gray humic acids have a marked colloidal character, a large number of surface functional groups, and are subject to aggregation phenomena. They are able to complex soluble pollutants, and initiate flocculation processes as a function of environmental conditions. The aim of this work is to study the aggregation of a gray humic acid, which is stable in colloidal dispersion, by means of photon correlation spectroscopy, and molecular modeling. The effect of this aggregation in the solid state is also studied by means of N2 (to 77 K) and CO2 (to 273 K) adsorption isotherms, as well as FT-IR absorption. The variation of the colloid's zeta potential and size, with pH, reflects the ionization of the carboxylic and phenolic acidic groups, and a linear dependence of size on zeta potential. The decrease in the size of the colloids seems to be more affected by the ionization of the phenolic acid groups, than by that of the carboxylic acid groups, which is likely because in the case of the ionized carboxylic groups the humic colloids are still capable of generating H-bonds. In the solid state, aggregation effects are illustrated by a decrease in surface area, and a disappearance of certain micropores, with increasing pH. These features are likely due to an inhibition of aggregation in the colloidal state as a consequence of the increase in charge that results from ionization of the acidic groups, and also to an increased hindrance to H-bond formation, due to the loss of protons during the above-mentioned ionization process.

Surface-enhanced Raman scattering for ultrasensitive chemical analysis of 1 and 2-naphthalenethiols.

The results of the search for the optimal experimental conditions for ultrasentitive chemical analysis of 1-naphthalenethiol (1-NAT) and 2-naphthalenethiol (2-NAT) using surface-enhanced Raman scattering (SERS) are discussed. The report begins with a review of the vibrational spectra, including infrared and Raman spectra of the target molecules, and the interpretation of the observed frequencies aided by local density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level of theory. Several metal nanostructures were tested for SERS activity, including island films and colloids of silver, gold and copper. Correspondingly, the most effective laser line for excitation in the visible and near infrared region was sought. The achieved detection limit for 1-naphthalenethiol, and for 2-naphthalenethiol, on silver nanostructures is in the zeptomole regime.

Modeling the adsorption and precipitation processes of Cu(II) on humin.

Humins (HU) are the most insoluble fraction of humic substances. Chemically, they can be considered as humic macromolecules bonded to the mineral matter of soil. The HU have a marked colloidal character and they are extremely important in retention of pollutants in soils. The aim of this work is to combine adsorption data with spectroscopic techniques in order to study the adsorption and precipitation processes of Cu(II) on HU. Analysis of sorption isotherms by means of several single-adsorption-process-based models makes it possible to obtain the speciation diagrams of Cu(II) species on HU surfaces. Further, FTIR (which provides information about the changes in the surface groups of the HU) and DRX (which shows the formation of possible crystalline phases on the HU surface) were used to determine the specific interactions of Cu(II) cations with the surface reactive groups of HU. The shape of the isotherms at constant pH varies with pH from L1-type (pH 2-4) to L3-type (pH 5-6) and S-type (pH 8), which indicates a change in the retention mechanism. When pH is 2 the retention of Cu(II), as [Cu(H(2)O)(6)](2+), is the preferred retention mechanism. The retained quantity of Cu(II) as [Cu(OH)(H(2)O)(5)](+) increases with pH. Starting from pH 4 the Cu(II) begins its precipitation, which is the preferred retention mechanism at pH 8. The presence of HU has a great influence on the precipitation process of Cu(II), giving rise to botalackite, which reveals epitaxial growth of crystals.

Cu(II) retention on a humic substance.

Humic substances (HS) are macromolecular products derived from a physical, chemical, and microbiological process called "humification." These substances play an important role in the mobility and bioavailability of nutrients and contaminants in the environment. Adsorption isotherms provide a macroscopic view of the retention phenomena. However, complementary techniques are needed in order to study the retention mechanism. The application of the classical models and some modern ones, based on humic substances chemistry, do not accurately describe these adsorption data. The aim of this paper is to model isotherms and combine adsorption data with spectroscopy and microscopy techniques to study the Cu(II) retention on a HS. The adsorption isotherms shape varies significantly with the solution pH from L-type (pH 2-6) to S-type (pH 8). FTIR shows that, when pH is 2 the retention of Cu(II), as [Cu(H(2)O)(6)](2+), is the preferred retention mechanism. The quantity of Cu(II) retained as [Cu(OH)(H(2)O)(6)](+) rises, as pH increases. At pH 4, Cu(II) begins to precipitate, which is the preferred mechanism at pH 8.02. The presence of HS has a great influence on the precipitation process of Cu(II), giving rise to amorphous precipitates. As it is shown by SEM-XRF, Cu(II) distributes heterogeneously on HS surface and accumulates on the humic phases. The presence of different anions (chloride and nitrate) slightly modifies the HS behavior as cation exchanger. When Cl(-) ions are present, part of the Cu(II) form [CuCl(4)](2-), which is stable in solution due to its negative charge; when the anion present is NO(3)(-) the formed complex, [CuNO(3)](+), is retained on the HS.