Research - Dyeing Crystals
Crystals of Many Colors
Dyeing Growth Spirals
Origin of Life
Single Photon Sources
For a number of years we have been studying the art and science of dyeing crystals, a very general and beautiful aspect of supramolecular chemistry. Graduate student Jason Benedict has been engaged in studying crystals that precipitate from solutions wearing multicolored suits. He has completed a study of diaminoacridine in potassium hydrogen phthalate (KAP for potassium acid phthalate) crystals that emit light of three distinct energies from growth sectors not related by symmetry. The identity of the various luminescent species was aided by Marina Brustolon's (Padua) analysis of the crystals using time-resolved EPR spectroscopy. The pH indicator methyl red gives only bi-colored phthalic acid crystals but it took years to sort out the chromochemistry of this system. This work, a synthesis of crystal growth studies, polarized light microscopy, and computations carried out in collaboration with Andrew Rohl (Curtin Institute, Perth), has debunked, at least in this one case, the vexing concept of syncrystallization that persisted in the literature 100 years ago. Jason has also used dyes to distinguish polymorphs. A particularly intriguing example is the use of methyl red to distinguish polymorphs of 2,5-dihydroxybenzene, a common matrix for MALDI mass spectrometry. Dawn Cohen has taken on a crystallographic assessment of the ionization mechanism in MALDI MS. These experiments mark the beginning of our search with Daniel Chiu, for a correlation between crystal chemistry and the MALDI ionization process.
Figure 1. Dyes selectively staining potassium hydrogen phthalate (KAP) crystal hillocks.
Spiral growth in crystals at low supersaturation results in topographical features known as hillocks, shallow stepped pyramids. Impurities may inhomogeneously deposit within a single growth sector due to selective interactions between impurities and the distinct vicinal hillock slopes. The patterns of light that result from intrasectoral zoning of some luminescent dyes in KAP are shown in Figure 1. Fluorescent dyes incorporated into the crystal throughout the growth process create a 'fossil-record' of hillock evolution in patterns of light that can be 'dug-out' with a confocal luminescence microscope. Graduate student Theresa Bullard has been devoted to this 'paleontology'. She is employing machine vision algorithms for automated visual inspection and image analysis of hillocks through a multi-layered procedure of graphical processing, data reduction, and image interpretation. She then uses fractal analysis to quantify the spatio-temporal distribution of hillocks and their evolution, determining whether the screw dislocation cores throughout the crystal are randomly distributed, fractal-like, or correlated with one another. By growing crystals under controlled conditions of varying temperature, supersaturation, and with additives, Theresa aims to identify the mechanisms that give rise to growth active hillocks.
Ultimately, we want to record the 'endocytotic' process whereby the propagating steps engulf the unlikely impurities. We are currently producing a 'movie' using confocal scanning laser microscopy (CSLM) to show the localization of single, large molecular luminophores on crystallographic steps while at the same time mapping out step topography and propagation kinetics by atomic force microscopy (AFM). As the luminescent guest is invisible to the AFM and step propagation is invisible to the confocal microscope, images with each technique must be made simultaneously. This requires the construction of a hybrid CSLM/AFM (Figure 2), which postdoc Ryan Sours is undertaking in collaboration with James De Yoreo and Chad Talley at Lawrence Livermore National Laboratory (LLNL).
Figure 2. Simultaneously acquired atomic force (A) and confocal fluorescence (B) micrographs of 26 nm diameter fluorescent polymer microbeads deposited on the {010} face of a KAP crystal. Scale bar = 10 μm. Schematic of the AFM/CLSM instrument used to collect the micrographs (C).
An unlikely application of the dyeing of crystalline hillocks is in origin of life science. Cairns-Smith (Genetic Takeover and the Minerals Origins of Life, Cambridge University Press, Cambridge, 1982) presented a revolutionary view of life's origins whereby mineral genes with 'information' stored in patterns of crystal defects transfer information from one crystal to another via fragmentation and epitaxy. His ideas have captured a place in the origin of life community, but the acknowledgments of their virtues are invariably accompanied by the caveat that they have resisted experimental support. We have embarked on an experimental test of Cairns-Smith's 'crystals-as-genes' hypothesis in a model crystal of KAP that unlike comparatively messy clay minerals, has characteristics that enable us to clearly frame the ideas of Cairns-Smith in terms of reproducible laboratory experiments. Of the many mechanisms whereby crystals might store information, Cairns-Smith emphasizes the screw dislocation. A crystal with many parallel screw dislocations can be thought of as a punched computer card with information encoded as the spatial disposition of defects. By cleavage perpendicular to the dislocation cores, new surfaces emerge having complementary high-energy sites that can then continue to grow independently. With luminophore and confocal microscopy John Freudenthal can map the birth and death of growth hillocks, evaluate the information content contained within their spatial disposition using the mathematics of fractals, and investigate the transfer of this information via cleavage and subsequent growth.
Figure 3. Crystal of KAP cut parallel to (010) through five screw dislocations. Complementary halves grown in the presence of a hillock selective organic luminophore should show complementary patterns of luminescence. The experiment shows that much information is preserved (white triangles) but that there is also a considerable amount of mutation (light blue triangles) in the first generation. This can be minimized by refining our technique.
Single crystals with oriented chromophores are ideal objects for single molecule spectroscopies. Graduate student, Kristin Wustholz, jointly supervised with Phil Reid, has completed the first systematic single molecule study of dyed crystals focusing on orientational distributions in KAP [Figure 4]. This work led to the realization that dyed crystals are ideal single photon sources, vital materials in quantum information processing and quantum cryptography. Currently implemented, single-photon sources cannot be made to produce single photons with high probability, while simultaneously suppressing the probability of yielding two or more photons. Our dyes in single crystals are remarkably stable emit polarized light, and are cheap and easy to prepare. This augurs well for their use as deterministic single photon sources. Eric Bott is building a time-correlated single photon counting apparatus to test these ideas.
Figure 4. Confocal fluorescence image (10 μm 10 μm, left panel) of single molecule embedded in a KAP crystal. The encircled molecule exhibits determined luminescence under cw excitation at 532 nm of ~1.7 μW for several minutes.