Light-sensitive gel based on cyclic 'pumpkins'

Macrocyclic cavitands such as cyclodextrins or calixarenes having hydrophilic differences between inside and outside the cavities often show a binding or inclusion ability toward a wide variety of molecules to give a supramolecular construct called inclusion complex, provided the sizes of the guests match the cavities of the hosts. The inclusion complex is formed non-covalently and thus can be dissociated reversibly, under certain condition. This reversible feature has enable scientists to develop various stimuli-responsive supramolecular-structured gels. Cyclodextrins, for example, have been employed with well defined block or graft copolymers to form hydrogels with sensitivities to a number of environmental stimuli^†, which is the direction of my graduate thesis. Host-guest interaction is thought to be the driving force of the physical cross-linking responsible for the gelation. Cucurbituril, named by its pumpkin-like morphology, is another family of macrocyclic molecules. Comprising n glycoluril units, cucurbit[n]urils (n=5-10) have a variable-sized cavity that is accessible through two identical carbonyl-fringed portals.

Cucurbit[7]urils (CB[7]), whose cavity size is comparable to that of β-cyclodextrin, has modest solubility in water. Kimoon Kim, who is one of the most active supramolecular chemists, and co-workers recently reported a hydrogel based on only CB[7], a host molecules, via self-assembly in acidic conditions (Angew. Chem. Int. Ed. 2006. DOI: 10.1002/anie.200603149). The CB[7] gel shows a slow gel-to-sol transition over a temperature range of 43-57°C and a sharp sol-to-gel transition at around 42°C. Characterized by AFM and small-angle X-ray scattering (SAXS), it is found that the CB[7] molecules self-assembly into unit 1D fibrils, which further align into thick bundles of fiber with lengths up to several tens of micrometers and form a network (figure on the right^‡, click to enlarge). Addition of proper guest molecules, however, may induce further stimuli-responsive behavior into the CB[7] gel. 4,4'-diaminostilbene dihydrochoride, a light sensitive molecule that shows photoisomerization, is known to form stable 1:1 inclusion complexes with CB[7] at both trans and cis conformations. The guest-including CB[7] molecules can thus self-assembly into a gel under certain condition. When irradiated by UV light (365nm), the gel turns into yellow sol; heating the sol follwed by cooling induce the gelation again. Therefore, by choosing the appropriate guest molecules, one may be able to design various stilmuli-responsive gel systems based on the intriguing host-guest chemistry of CB[7] and the properties of the included guest molecules.

† For example, Biomaterials 2006 27 4132-4140; Macromolecules 2005 38 5223-5227; Macromol. Rapid Commun. 2006 27 238-241; Macromolecules 2006 39 2614-2620.

‡ Adapted from the original paper. Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We can grow a...tooth?

Nature provides us a wide variety of beautiful structures and functionalities via incredibly high level of self-assembly, which has been inspiring scientists and engineers for decades. In particular, the hard tissues of organisms (shells, bones, teeth, etc.), often composite materials of organic and inorganic components, resemble a heated subject in both biological and material research. These hard tissues, or biominerals, contain crystals whose shapes are very different from the crystal habits produced inorganically. Large-scale and defined crystal structures are hardly found in biominerals; instead, the crystals may be 'molded' into elaborate structures which have non-faceted crystal surfaces, wherein the 'mold' is a vesicular compartment within which the crystal is formed. The ability to 'mold', or grow such energetically unfavorable structures inorganic components that cannot be melted at physiological temperatures has deeply intrigued the materials engineering community. Understanding the process of biomineralization may find potential application in low-temperature processing techniques in materials industry.

Biomineralizaion is controlled at multiple levels, including the regulation of particle size, shape, crystal orgientation, polymorphic structure, defect texture, and particle assembly. Formation of complex composites with hierarchical composite structures involves all level of biological elements, from bio-macromolecules to cells. Insights into the mechanisms of biomineralization, thanks to biology, enable materials scientists to develope similar techniques that either mimic (biomimetic) or are inspired by (bio-inspired) biological processes. Although it would be difficult for scientist to mimic complex cellular processes, however, the materials chemistry aspects of biomineralization can be studied by model systems, and utilized for biomimetic engineering.

One particular aspect of interest to the materials chemist is the means by which these organisms use organic constituents (macromolecular templates, vesicular compartments, solubilized peptides or proteins, etc.) to mediate the growth of the mineral phase. In particular, with the development of the controlled polymerization techniques such as atom transfer radical polymerization (ATRP) or reversible addition-fragmentation transfer (RAFT) polymerization and sophisticated understanding of the properties of polymers, synthetic macromolecules have been recognized as a promising route to direct the growth of defined crystal structures of inorganic materials.

In a recent report by Dirk Volkmer and co-workers (Angew. Chem. Int. Ed. 2006 45 7458-7461. DOI: 10.1002/anie.200602382) described how transient amorphous calcium carbonate (ACC) can be stabilized by ionotropic polymethacrylic acid (PMAA) brushes with defined thickness on glass surfaces, and transformed into polycrystalline calcite films with similar thickness of the brush by a simple thermal treatment. Then a glass substrate covered with patterned PMAA brushes fabricated via a TEM grids as photomasks under Hg-Xe irradiation was tried (see the scheme on the right^†). Again, polycrystalline calcite with similar thickness and patterns to the precursor polymer brush was obtained.

The calcite thin film is an exact 3D replica of the ionotropic PMMA matrix, which means the thickness of the film can be adjusted by varying the length polymer brush, which can be easily controlled owing to the 'living' nature of ATRP. Furthermore, in addition to photolithography, a 'top-down' approach, 'bottom-up' approaches such as SAMs or self-assembly of copolymers provide more patterning possibilities. The transient ACC-process of biomineralization of calcium carbonate has been regarded as the most possible mechanism of the formation of various complex biogenic calcite structures such as sea urchin spines. It has also been exploited as a transient precursor in vitro to produce CaCO₃ crystals in constrained geometries, to fabricate microstructured calcite single crystals, and to synthesize nacre-type laminated CaCO₃ coatings. This approach could lead to potential fabrication of novel inorganic or composite materials with interesting optical, catalytic, or biological properties; and presumably, we will be able to grow a tooth with 'real' quality and pre-designed shape. Which do you like, vampire or rabbit style?

† The figure was adapted from the original paper. Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Doctor Idiots

A late post in the blog Atom Pushers Anonymous talked about a Chinese Ph.D and loser, who failed even understanding the basics of organic chemistry. As a grad in China I can well confirm the perplexity of Atom Pusher - Yes, we have many losers here who successfully got their PhD and went abroad!

I know a PhD student who sleeps in every class. He asked his MS collegue what's anti—Markovnikov addition! I also know another PhD student who knows nothing about MS Word. 'How did you prepare your MS paper?' I asked with deep perplexity. 'Well, I just had it prepared.' he mummered.

Right! In China you always have more than one way to get what you want, including PhD, MBA and whatever. The most important thing here is guanxi rather than professionalism.

Many in China have been ruined by guanxi and what it is derived and derives. Higher education is a case. I suggest institutes abroad consider more carefully whenever before a Chinese candidate, and limit, dismiss or refuse one without any hesitation whenever necessary.

Reproducible? Scientific? - Comment on 'Scientists on Science: Reproducibility'

Nobel Intent's new article, Scientists on Science: Reproducibility, has raised heated discussion among bloggers. It is possible and favorable to summarize another 'Bloggers on Science: Reproducibility' or something like that. So here, I try to contribute to this project with my comprehension of science.

One of the purpose and perhaps the most essential one of science is to know the truth of the nature. Unfortunately, truth never shows itself directly and completely. So we observe the nature and get some observations. It is here that the issue of reproducibility rises - this word is actually used to describe the observations of the nature, no matter man-triggered or natrually occuring - nature.

Observation reflects nothing but the truth about the nature, unexceptionally, but never completely or directly, and sometimes twistingly and misleadingly. Science, providing a set of methodology such as logics and statistics, translates or comprehends the observations into knowledge it belives about the truth, at least partially, and tests the knowledge by more observations under this methodology again (deduction, experiment, etc.). Scientists belive that this methodology can distinguish the false observations (those which reflect the truth misleadingly) from true ones^†. They are also aware enough that the observations are always specific (not completely reflecting the truth). Scientists feel nothing uncomfortable if truth is in constant variation, and the knowledge of the truth is revisable and frequently revised, but they always revise former knowledge based on new observation comprehended by that methodology, again. They have seldom, if not never, revised the established this methodology up till now.

Reproducibility is only one facet of observation. It is one of the factors by which scientists consider how and to what extent the observations reflect the truth. Reproducibility is now considered important because currently the methodology to comprehend the observation rely badly on reproducibility. This methodology worked well before, because nature did show us reproducible events when we tried it. But nowadays, with the our observations developing and deepening, especially in quantum mechanics, the old methodology based on definitude of logics, smmetry of time and space, or clear convergence of statistics that scientists have been relying on seems to be increasing doubtful, and new philosophy and methodology has been raised and discussed.

Although science has troubles in its own field, as it always does, there is still none of religion's business here, though, becuase the purpose of religion is never to know the truth, and it does not base its knowledge on observations like science does. Religion deals with people. It is in constant need of more people who belive in its pre-established creed and the stories or theories that support the creed, which it belives can salvage the believers. The totally differences in initial purpose of religion from science stop us from comparing religion and science at any level. Equally absurd is trying to diminish science by claiming religion is more likely the truth, or vise versa, especially in the modern time when people are entitled to belive whatever they wish to. The only comparable difference between science and religion, in my opinion, is that scientists always have new things to 'play' with; they can trigger some events, get some observation, and traslate them into knowledge; they can also improve the tools so that an enlarged scope of nature can be observed. Due to the infiniteness of the universe they can just keep doing so forever, during which process they feel even more content when some translations can last long. Religion, in contrast, provides less or no new 'toys' by itself; it thus turn to science, coupling the latest scientific findings with their old creeds so as to support their existence more strongly. It does so only because an increasing fraction of the modern public are science believers and tend to judge everything by observations; religion is only catering to this tendency, and in addition mal-traslating new observations into theory that support their claims, in order to enlarge the number of their believers. As was mentioned, science deals with observation while religion deals with people. Although they interweave each other in the modern time, they should never be conflicting.

Talking back to the issue of reproducibility, another reason why it is considered so imporant lies in another purpose of science - improving our lives. To do so, we need technologies - something need to be reliable and hence reproducible - and batch production, which equally much relies on reproducibility. After science have provided us with Internet, polyethylene, etc., we took it for granted and asked for more. This additional responsibility loaded on science renders reproducibiliy a more essential issue, so much that almost every evaluation in scientific results is at least partly application-oriented where reproducibility is the fist point to fulfill. Under these circumstances, Scientists have learned to care about not only finding the truth, but also finding something reproducible - even regardless of they are true or not! This is another reason why reproducibility become such an exclusive criteria of science, and is abused to judge, critize, and deny scientific reports both by scientists and by religionists.

As human, we only care about the improvement of our own lives, both materially and spiritually. But some people, unconsciously, hope that these two aspects of their lives to be improved by only one power, be it religion or science, for either of the two has been seemingly so powerful. Unfortunately, neither religion nor science sets its goal to improve both aspects of human life, while either of them is able to improve one pretty well.

Do you belive the nature is knowable? If you do, do you think it is important to know the nature? I belive more people than expected would give negative answers to these questions, even among scientists. They feel science is important only because it provides for instance digital cameras or plasma TVs. Science ought to provide plasma TVs, in their opinions, otherwise science should be of no use, false, or even cheating. They are actually using science instead of interesting in it, and things like plasma TVs are the only field that religion can do nothing, therefore science is worthy if it does well or worthless if it fails. In fact, we are not forced by anyone to choose between science and religion as our salvager. There is no one but you can salvage youself, while religion and science can be of some help but are never omnipotent.

† This belief is derived from the philosophy that the world is knowable by human. Some philosophies however do not think so and do not support the rationality of science. Religion is among the latter.