Strategies for in-liquid molecular detection via Surface Enhanced Raman Scattering (SERS) are currently based on chemically-driven aggregation or optical trapping of metal nanoparticles in presence of the target molecules. Such strategies allow the formation of SERS-active clusters that efficiently embed the molecule at the “hot spots” of the nanoparticles and enhance its Raman scattering by orders of magnitude. Here we report on a novel scheme that exploits the radiation pressure to locally push gold nanorods and induce their aggregation in buffered solutions of biomolecules, achieving biomolecular SERS detection at almost neutral pH. The sensor is applied to detect non-resonant amino acids and proteins, namely Phenylalanine (Phe), Bovine Serum Albumin (BSA) and Lysozyme (Lys), reaching detection limits in the μg/mL range. Being a chemical free and contactless technique, our methodology is easy to implement, fast to operate, needs small sample volumes and has potential for integration in microfluidic circuits for biomarkers detection.

A structural change from fractal to nanorod J-aggregates of tetrakis(4-sulfonatophenyl)porphyrin has been obtained by acting on the intermolecular interaction potential. The size and shape of these self-assembled porphyrin clusters have been monitored under different experimental conditions, by means of polarized and depolarized dynamic light scattering and small and wide angle elastic light scattering. At sufficiently low porphyrin concentration and high ionic strength, the shielded repulsive potential seems to be responsible for the fractal structure of the aggregates. On the contrary, at low ionic strength (nonshielded potential) and high porphyrin concentration, these species self-assemble in a rodlike arrangement. The length of the so-formed rod-shaped aggregates decreases on increasing porphyrin concentration. Moreover, both fractals and rods display a structure-dependent optical activity induced by a chiral template.

Aqueous solutions of poly(ethylene oxide) were investigated using the ultrasonic technique, photon correlation spectroscopy (PCS) and nuclear magnetic resonance (NMR), in a wide range of molecular weight (from ethylene glycol to poly(ethylene oxide) 4 000 000 Da). Ultrasonic data reveal that the mixing process is not ideal and show that the polymer–water interaction strength increases with the polymerization degree. PCS and NMR, on the other hand, furnish a free particle diffusion coefficient which satisfies a unique scaling law from 8000 to 4 000 000 Da and demonstrates the good solvent nature of water. These experimental findings indicate that polymer–polymer aggregation processes are not an inherent property of these systems.

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We have investigated in detail the self-assembly of a chiral porphyrin trimer in different solvents and correlated this behavior to the aggregation of the molecule at a solid-liquid interface. In n-hexane and cyclohexane, CD spectroscopy and dynamic and static light scattering studies showed that the porphyrin trimer self-assembles already at micromolar concentrations into long, chiral supramolecular polymers, which precipitate as fibers when the solution is drop-cast onto a mica surface. In contrast, in chloroform, the compound is molecularly dissolved up to concentrations of 0.2 mM and when micromolar solutions are drop-cast onto mica, no precipitation of large assemblies occurs. Instead, at the moment that the chloroform film becomes subject to spinodal dewetting and the porphyrin trimers within this film start to self-assemble, extended patterns of equidistant lines of single molecule thick columnar stacks are formed.

Recently, we showed that J-aggregates, formed by the zwitterionic diacid form of the porphyrin 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS4 4-) under acidic conditions, can be described in terms of fractal geometry. We have extended the investigation of the system under strongly acidic conditions through a combination of UV/vis and fluorescence emission spectroscopy, together with static (SLS), quasi-elastic (QELS), and resonant (RLS) light-scattering techniques. The experimental findings suggest that porphyrin J-aggregates begin to form at pH 1 and are stable up to 4 M HCl. On further increasing the acid concentration, protonation of the sulfonate end groups occurs, leading to a disruption of the electrostatic interactions between these anionic groups and the charged protonated nitrogen atoms in the porphyrin core. At [HCl] > 8 M, the spectroscopic features suggest the presence of the monomeric dicationic porphyrin, resulting from a full protonation of the four sulfonate groups. Under the acid concentration range in which J-aggregates exist, light-scattering data indicate the formation of clusters having a fractal internal structure. At [HCl] = 0.1 M, the aggregation is driven by the interaction between small clusters leading to a loose diffusion-limited cluster−cluster aggregation (DLCCA) structure (d f = 1.75 ± 0.05). On increasing the acid concentration up to 2 M HCl, a structural crossover occurs. The reduction of the net charge on the monomeric unit leads to an increased sticking probability between monomers which is responsible for the observed compact diffusion-limited aggregation (DLA) structure (d f = 2.5 ± 0.03). A further reduction of the net charge of the porphyrin ([HCl] = 4 M) determines the formation of nucleating clusters no longer having a fractal structure. An important role is played by the mixing order of the reagents both at the level of kinetics of growth and for the final mesoscopic structures. Our findings suggest that this effect should be related to the large volumetric ratio between the reagent solutions to be mixed, which causes very different spatial concentrations of both reagents.

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The present paper reports on the study of the disaccharides' dynamics in aqueous solutions and discusses the role of their interaction with water, which seems to be responsible for the protective action on biological membranes and tissues. In particular, using QENS (IRIS at ISIS, RAL, U.K.) we compared the quasielastic spectra from three homologous disaccharides (trehalose, maltose, and sucrose) with a hydration of 20 water molecules per each disaccharide molecule. The isotopic substitution method allowed us to study also the dynamics of water in the presence of the disaccharides. Our results show that trehalose is the most effective in slowing down the water dynamics, inducing a more extensive hydration layer.