Adsorption study and detection of the high performance organic pigments quinacridone and 2,9-dimethylquinacridone on Ag nanoparticles by surface-enhanced optical spectroscopy.

In this work, surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF) were employed in the study of the adsorption and detection of the pigments quinacridone (QA) and 2,9-dimethylquinacridone (2,9-DMQA). These pigments are of great relevance in artwork and textile, plastic, and photochemical industries due to their condition as high performance pigments since they possess a high tinting strength. Due to this fact, they have been employed at relatively low concentrations. Therefore, the analysis and detection of these pigments requires the application of a highly sensitivity technique, such as SERS and SEF. The adsorption of QA and 2,9-DMQA on silver nanoparticles was extensively studied by means of SERS at different surface coverages. This study was completed by carrying out an in depth vibrational (Raman and IR) analysis of these pigments in solid state by ab initio density functional theory (DFT) calculations. In addition, UV-vis spectroscopy was employed to investigate the aggregation undergone by both pigments in solid state and in solution. 2,9-DMQA was demonstrated to have a lower tendency toward aggregation due to the presence of methyl groups. Even so, this molecule follows a BET adsorption mechanism on the metal nanoparticles due to its high tendency toward intermolecular interaction. From the analysis of the adsorption mechanism of this molecule, the limit of detection was deduced to be ca. 55 ppb.

Effect of metal-liquid interface composition on the adsorption of a cyanine dye onto gold nanoparticles.

Synthesis of asymmetric nanoparticles, such as gold nanorods, with tunable optical properties providing metal structures with improved SERS performance is playing a critical role in expanding the use of SERS to imaging and sensing applications. However, the synthetic methods usually require surfactants or polymers as shape-directing agents. These chemicals normally remain firmly bound to the metal after the synthesis, preventing the direct adsorption of a large number of potential analytes and often hampering the chemical functionalization of the surface unless extended, and critical for the nanoparticle stability, postremoval steps were performed. For this reason, it is of great importance for the full exploitation of these nanostructures to gain a deeper insight into the dependence of the analyte-metal interaction to the metal-liquid interface composition. In this article, we investigated in detail the role played by each component of the gold nanorod (GNR) interface in the adsorption of indocyanine green (ICG) as a probe molecule. Citrate-reduced gold nanospheres were used as a model substrate since the negative citrate anions adsorbed onto the metal surface can be easily displaced by those chemicals usually involved in the GNR synthesis, allowing the GNR-like interface composition to be progressively rebuilt and modified at will on the citrate-capped nanoparticles. The obtained results provide a meticulous description of the role played by each individual component of the metal-liquid interface on the ICG interaction with the metal, illustrating how apparently minor experimental changes can dramatically modify the affinity and optical properties of the ICG probe adsorbed onto the nanoparticle.

Electrochemical SERS study on a copper electrode of the insoluble organic pigment quinacridone quinone using ionic liquids (BMIMCl and TBAN) as dispersing agents.

SERS detection of quinacridone quinone (QAQ), an insoluble synthetic organic pigment relevant to modern artworks, is reported here. The use of ionic liquids (BMIMCl and TBAN) as dispersing agents has allowed us to carry out electrochemical SERS experiments of QAQ in aqueous solution using a Cu electrode. No SERS spectra were obtained either from the ionic liquids (ILs) or from QAQ when silver/gold colloids were employed. The spectra of the Cu electrode in 0.1 M KCl aqueous solutions containing either BMIMCl or TBAN indicate that the corresponding cations approach the metal surface through formation of an ion pair with specifically adsorbed Cl⁻, but the corresponding cyclic voltammograms only show reduction and oxidation of the Cu surface. In the case of QAQ dispersed in BMIMCl and TBAN, two different SERS spectra due to QAQ species are observed between -0.4 and -1.0 V. One of the spectra agrees with the SERS of QAQ previously reported by us, using calixarenes as dispersing agents and silver colloids as SERS substrates. The other spectrum reveals a reallocation of the charge of the QAQ molecule, concomitant with a change in its relative orientation to the Cu surface. The SERS detection of QAQ is free from interferences of bands corresponding to the ILs.

Solution SERS of an insoluble synthetic organic pigment-quinacridone quinone-employing calixarenes as dispersive cavitands.

A possibility of getting SERS spectra of insoluble aromatic compounds in colloidal silver solutions is described. The method tested for the organic pigment quinacridone quinone consists of dispersing it in calix[n]arenes. The potentials of such cavitands, both as dispersing and as silver functionalization agents, is reported as a function of the substitution in their lower rim and their cavity size.