The blinking and photobleaching dynamics of alizarin (1,2dihydroxyanthraquinone) and purpurin (1,2,4-trihydroxyanthraquinone) are investigated using single-molecule spectroscopy. The time-dependent emission of alizarin and purpurin on glass under N 2 is analyzed using the change point detection (CPD) method to compile on-and off-event distributions. The number of distinct emissive events per molecule is about four times higher for alizarin relative to purpurin, consistent with an excited-state intramolecular proton transfer (ESIPT) process to populate an emissive tautomer state. To elucidate the mechanism for blinking (i.e., switching between on and off events), maximum likelihood estimation (MLE), goodness-of-fit tests based on the Kolmogorov−Smirnov (KS) statistic, and the loglikelihood ratio (LLR) tests are used to establish the best fits to the on-and offinterval probability distributions. For both alizarin and purpurin the on intervals are log-normally distributed, and off intervals are Weibull distributed, consistent with a dispersive electron-transfer (ET) kinetics model for blinking (i.e., involving Gaussian-like distributions of activation barriers to ET). Further analysis of the blinking dynamics reveals that ET to a long-lived dark state most often precedes molecular photobleaching, where extended residency in the dark state increases the probability of photobleaching. Based on these findings, mechanisms for the blinking and photobleaching of alizarin and purpurin are proposed. The ability of alizarin to undergo ESIPT enables fast excited-state decay and decreases the probability of ET. In contrast, purpurin exhibits faster injection and slower back ET relative to alizarin, leading to increased photobleaching via a dark radical cation state.
Surface-enhanced Raman scattering (SERS) studies of art represent an attractive way to introduce undergraduate students to concepts in nanoscience, vibrational spectroscopy, and instrumental analysis. Here, we present an undergraduate analytical or physical chemistry laboratory wherein a combination of normal Raman and SERS spectroscopy is used to identify both inorganic and organic fluorescent colorants in an oil painting. On the basis of their experimental observations, students make procedural decisions to adjust acquisition settings and use SERS, thereby enabling the successful identification of unknowns. This laboratory engages undergraduate students by applying what they have learned about quantum mechanics, nanoscience, and spectroscopy to the real-world, problem-solving context of art conservation. S urface-enhanced Raman scattering (SERS) spectroscopy has become a powerful technique in analytical and physical chemistry. By integrating nanoscience with vibrational spectroscopy, SERS provides for the unambiguous and ultrasensitive detection of a wide variety of analytes. 1 For example, SERS is increasingly applied to the field of art conservation to identify colorants in minute samples from cultural heritage objects. 2−8 Indeed, SERS studies of artists' materials represent an excellent method to familiarize undergraduate students with modern applications of spectroscopy and nanoscience. Various experiments have been presented to introduce undergraduates to the field of art conservation using Raman spectroscopy, 9 UV/vis absorption, 10 NIR imaging, 11 and XRF. 12 Similarly, several laboratories have been developed to demonstrate the SERS effect and estimate enhancement factors, 13−16 but there are no existing experiments devoted to the real-world application of SERS to art conservation. The integration of SERS with art conservation enriches the learning experience by adding elements of nanoscience and advanced spectroscopy within the engaging context of art. Furthermore, this laboratory simulates a problem-solving scenario for students to detect unknown colorants in art. Students encounter problems throughout this experiment that require generating testable hypotheses and making new procedural decisions. In this problem-based learning (PBL) approach, 17 the instructor serves as a problem-solving guide by asking questions and facilitating group discussion. Ultimately, the combination of nanoparticle synthesis, normal Raman spectroscopy, and SERS in this PBL experiment enables students to learn how to identify both inorganic and natural, organic pigments in small samples from an actual oil painting.In this experiment, students are presented with an Andy Warhol-inspired oil painting of four red flowers on a gray background. The composition for the painting is adapted from the print "Flowers" by Warhol to take advantage of the basic fields of color and to connect the experiment to an iconic element of art history. Each flower contains one of the following colorants bound in linseed oil: madder lake, lac dye, ca...
The analysis of paint cross-sections can reveal a remarkable amount of information about the layers and materials in a painting without visibly altering the artwork. Although a variety of analytical approaches are used to detect inorganic pigments as well as organic binders, proteins, and lipids in cross-sections, they do not provide for the unambiguous identification of natural, organic colorants. Here, we develop a novel combined surface-enhanced Raman scattering (SERS), light microscopy, and normal Raman scattering (NRS) approach for the identification of red organic and inorganic pigments in paint cross-sections obtained from historic 18th and 19th century oil paintings. In particular, Ag nanoparticles are directly applied to localized areas of paint cross-sections mounted in polyester resin for SERS analysis of the organic pigments. This combined extractionless non-hydrolysis SERS and NRS approach provides for the definitive identification of carmine lake, madder lake, and vermilion in multiple paint layers. To our knowledge, this study represents the first in situ identification of natural, organic pigments within paint cross-sections from oil paintings. Furthermore, the combination of SERS and normal Raman, with light microscopy provides conservators with a more comprehensive understanding of a painting from a single sample and without the need for sample pretreatment.
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