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<p><b>नया पृष्ठ</b></p><div>'''स्व-एकत्रण''' ऐसे प्रक्रम हैं जो बिना किसी बाहरी हस्तक्षेप के, अव्यवस्था की स्थिति में विद्यमान प्रणाली से, पूर्व-उपस्थित घटकों के स्थानीय स्तर पर अंतःक्रियाओं के द्वारा स्व-संयोजित होते हैं। स्व-एकत्रण स्थैतिक या गतिक हो सकते हैं। स्थैतिक स्व-एकत्रण में किसी प्रणाली में संतुलन आता है, जिससे उसकी मुक्त उर्जा में कमी आती है। <!--whereas inpatterns of pre-existing components organized by specific, local interactions, and are not commonly described as "self-assembled" by scientists in the associated disciplines. These structures are better described as "self-organized".--><br />
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== रसायन विज्ञान और पदार्थ विज्ञान में स्व-एकत्रण ==<br />
स्व-एकत्रण को एक प्रणाली में [[अणुकणिका|<span title="molecules">अणुकणिकाओं</span>]] के [[गैर सह-संयुज बोन्डिंग|<span title="noncovalent bonding">गैर सह-संयुज अंतःक्रियाओं</span>]] के द्वारा स्वतः और उत्त्क्रमात्मक रूप में एकत्र होने को कहते हैं। इस परिभाषा में स्वतः एकत्रण को स्वतः रचन से भेद करना जरूरी है। उदाहरण के लिये, वर्षा को स्व-एकत्रण नही कहा जा सकता - इसके तीन कारण हैं:<br />
# स्व-एकत्रण में मुक्त [[ऊर्जा|उर्जा]] में कमी आनी चाहिये। अधिकांश [[रासायनिक अंतःक्रियाओं|<span title="Chemical Reactions">रासायनिक अंतःक्रियाओं</span>]] की दिशा, [[उष्मागतिकी|<span title="thermodynamics">उष्मगतिकी</span>]] के तहत, व्यवस्थित स्थिति से अव्यवस्था की ओर परिवर्तन होता है।<br />
# इसमें कम शक्ति वाले रासायनिक [[गैर सह-संयुज बोन्डिंग|<span title="noncovalent bonding">गैर सह-संयुज अंतःक्रियाएँ</span>]], जैसे [[हाइड्रोजन बाँड|<span title="Hydrogen bond">हाइड्रोजन बाँड</span>]], <span title="metallic coordination">धातु तालमेल</span>, [[जल विरोधी बल|<span title="Hydrophobic forces">जल विरोधी बल</span>]], [[वॉन डर वॉल बल|<span title="Vaan der val forces">वॉन डर वॉल बल</span>]], [[pi-pi अंतःक्रिया|<span title="pi-pi interactions"><math>\pi-\pi</math> अंतःक्रिया</span>]] और [[स्थिरविद्युत|<span title="electrostatics">स्थिरविद्युत</span>]], का महत्वपूर्ण भूमिका होती है। इनकी तुलना में [[सह-संयुज बोन्डिंग|<span title="covalent bonding">सह-संयुज अंतःक्रियाएँ</span>]], [[आयनिक बोन्डिंग|<span title="ionic bonding">आयनिक</span>]] और [[धातुविक बोन्डिंग|<span title="metallic bonding">धातुविक </span>]] बोन्डिंग शक्तिशाली होते हैं और ये द्रवों के भौतिक गुणधर्म, या ठोस पदार्थों के [[विलेयता|<span title="solubility">घुलनशीलता</span>]] पर प्रभाव डालती हैं, जिनका जीवित स्व-एकत्रित प्रणालियों (जैसे कि [[जीवाणु]] के [[कोशिका झिल्लियाँ|<span title="cell membranes">कोशिका झिल्लियों</span>]]) में महत्वपूर्ण भूमिका होती है। <br />
# स्व-एकत्रण के मूलभूत अंग सिर्फ [[अणु]] और अणुविकायें ही नही, पर कई तरह के [[नैनो]] ढांचे जो काफी बडे और अनेक आकृतियों में हो सकते हैं।<br />
स्व-एकत्रण के उदाहरण है [[आणुविक अभिज्ञान]] के द्वारा कार्यानवित [[प्रतिजैविक|<span title="Antibiotic">प्रतिजैविक</span>]] वैन्कोमैसिन, जिसका [[पेप्टाइड्|<span title="Peptides">पेप्टाइड्</span>]] जीवाणुओं के अंतक D-alanyl-D-alanine पर जुड जाता हैं, [[कलिल|<span title="colloids">कलिल</span>]], प्रावस्था पृथक्कृत [[पॉलीमर|<span title="polymers">बहुलक</span>]], इत्यादि।<ref name="whitesides_2002_pnas">Whitesides et al. 2002 PNAS</ref><ref name="whitesides_2005_science">Whitesides et al. 2005 Science</ref> <br />
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Therefore, we can say that SA extends the scope of chemistry aiming at synthesising products with order and functionality properties, extending chemical bonds to weak interactions and encompassing the self-assembly of NBBs on all length scales.<ref name="ozin_2005_nanochemistry">Ozin and Arsenault, Nanochemistry: a chemical approach to nanomaterials, (Cambridge: Royal Society of Chemistry), 2005</ref> In covalent synthesis and polymerisation, the scientist links atoms together in any desired conformation, which does not necessarily have to be the energetically most favoured position; self-assembling molecules, on the other hand, adopt a structure at the thermodynamic minimum, finding the best combination of interactions between subunits but not forming covalent bonds between them. In self-assembling structures, the scientist must predict this minimum, not merely place the atoms in the location desired.<br />
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Another characteristic that is common to nearly all self-assembled systems is their ''thermodynamic stability'': in order for SA to take place without the intervention of external forces, the process must lead to a lower Gibbs free energy, thus self-assembled structures are thermodynamically more stable than the single, unassembled components. A direct consequence is the general tendency of self-assembled structures to be relatively free of defects. An example is the formation of two-dimensional superlattices composed of an orderly arrangement of micrometre-sized polymethylmethacrylate (PMMA) spheres, starting from a solution containing the microspheres, in which the solvent is allowed to evaporate slowly in suitable conditions. In this case the driving force is capillary interaction, which originates from the deformation of the surface of a liquid caused by the presence of floating or submerged particles.<ref name="kralchevsky_langmuir_1994">Kralchevski et al., Langmuir, 2004</ref><br />
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These two properties&mdash;weak interactions and thermodynamic stability&mdash;can be recalled in order to rationalise another property which is often found in self-assembled systems: the ''sensitivity to perturbations'' exerted by the external environment: small fluctuations that alter the thermodynamic variables might lead to marked changes in the structure and even compromise it, either during or after SA. The weak nature of interactions is responsible for the flexibility of the architecture and allows for rearrangements of the structure in the direction determined by thermodynamics. If fluctuations bring the thermodynamic variables back to the starting condition, the structure is likely to go back to its initial configuration. This leads us to identify one more property of SA, which is generally not observed in materials synthesised by other techniques: ''reversibility''. <br />
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From what we have written so far, it should be evident that SA is a process which is easily influenced by external parameters: if this can make synthesis more problematic due to the many free parameters that require control, on the other hand it has the exciting advantage that a large variety of shapes and functions on many length scales can be obtained.<ref name="lehn_2005_science">Lehn et al, Science, 2005</ref><br />
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Generally speaking, the fundamental condition in order to have NBBs to self-assemble into an ordered structure is the simultaneous presence of long-range repulsive and short-range attractive forces.<ref name="forster_2002_angew">Forster et al, Angew. Chem., 2002</ref> Figure [fig_sa_scheme] exemplifies SA occurring from building blocks made up of two different units (A and B) which are covalently linked (short-range attraction) and repel each other by long-range interactions (e.g. because A is hydrophobic and B is hydrophilic). Since the energy of the system will unfavour configurations where A is close to B, and still no macrophase separation is possible due to A--B covalent bonds, the system will adopt a configuration where the contact area between A and B is minimised. This results in a periodic ordered structure (in figure [fig_sa_scheme] this is exemplified by a lamellar structure). For a list of repulsive-attractive competing forces which can give rise to SA phenomena, see<ref name="forster_2002_angew"/>.<br />
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By choosing precursors with suitable physicochemical properties, it is possible to exert a fine control on the formation processes in order to obtain complex architectures. Clearly, the most important tool when it comes to designing a synthesis strategy for a material, is the knowledge of the chemistry of the building units.<br />
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== उल्लेख ==<br />
<references/><br />
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== और देखें ==<br />
* [[स्व-रचन]]<br />
* [[नैनोप्रौद्योगिकी|नैनोतकनीकी]]<br />
* [[आणुविक अभिज्ञान]]<br />
<!--*[[Langmuir-Blodgett film]]<br />
*[[Autopoiesis]]--><br />
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== बाहरी कड़ियाँ ==<br />
* कुनियकी नागायामा, ''[https://web.archive.org/web/20080209210057/http://www.vega.org.uk/video/programme/70 विडियो 'प्रकृती में स्व-एकत्रण]''<br />
* लेख [https://web.archive.org/web/20040821022155/http://www.esi-topics.com/msa/ ''आणुविक अभिज्ञान'']<br />
* लेख [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=122665 ''अणुविकाओं के परे, विशाल घटकों में स्व-एकत्रण'']<br />
* वाईटसाइड्स, जी. एम. & ग्रजि़बॉस्की बी (2002) सायन्स 295, 2418-2421.<br />
* रोथमन्ड पी डब्ल्यु क, पापादाकिस एन, विनफ्री ई (2004) [https://web.archive.org/web/20090124105219/http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371%2Fjournal.pbio.0020424 ''डी एन ऐ की स्व-एकत्रण कलनविधि'']. PLoS Biol 2(12)<br />
* विकी: ''[https://web.archive.org/web/20110514000519/http://c2.com/cgi/wiki?SelfAssembly C2 स्व-एकत्रण का कंप्यूटर प्रोग्राम परिप्रेक्ष्य].''<br />
* पैलैस्को, जे ऐ, (2007) [https://web.archive.org/web/20150331040315/http://www.pelesko.com/ ''स्व-एकत्रण: वह विज्ञान जहाँ पदार्थ खुद ही जुट जाते हैं''] Chapman & Hall/CRC Press.<br />
* [https://web.archive.org/web/20171022162750/http://reza.nanodetails.com/ ''डी एन ऐ स्व-एकत्रण'']पर संक्षिप्त लेख <br />
* डेलावेयर विश्वविध्यालय का [https://web.archive.org/web/20070705160019/http://www.math.udel.edu/MECLAB/Projects/SelfAssembly/selfassembly1.htm ''स्व-एकत्रण''] पर संक्षिप्त लेख <br />
* मोहम्मदजा़देगन आर, शेकी एम एच (2007) ''[http://www.informaworld.com/openurl?genre=article&issn=0892-7022&volume=33&issue=13&spage=1071 ''डी एन ऐ नैनो-गियर] आणुविक अनुकरण 33(13); 1071-1081.<br />
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[[श्रेणी:जीव विज्ञान]]</div>imported>Escarbot