Patent Application: US-201314436785-A

Abstract:
the methods disclosed herein are directed to methods of enhancing formation of a polymer from a monomer on a metal - based nanoparticle under x - ray irradiation and compositions produced by such methods . the method comprises irradiating a monomer solution with x - rays to form the polymer ; wherein the monomer solution comprises the monomer , the metal - based nanoparticle , and a solvent capable of generating a hydroxyl radical ; and wherein the metal - based nanoparticle is a particle having a greatest dimension between 5 and 100 nanometers . the methods also include the dissolution metal ions from these same metal - based nanoparticles wherein the solution comprises the metal - based nanoparticle and a solvent capable of generating a hydroxyl radical ; and wherein the metal - based nanoparticle is a particle having a greatest dimension between 5 and 100 nanometers .

Description:
the following description presents the results of the first experimental study of the enhancement of polymerization of aniline enhanced by silver core - gold shell nanocoreshells ( ag @ au ncss ) under x - ray irradiation . the ncss afford both high sers sensitivity and minimal spontaneous formation of pani without x - rays . fig1 a shows a dark - field scanning transmission electron microscopy ( stem ) image of the dialyzed ncss . the ncss are clean and free of any small nanoparticles . upon irradiation with x - rays ( 100 kvp ) in the presence of only 0 . 1 mm aniline monomers , pani formed and was subsequently probed with sers . at this monomer concentration under x - ray irradiation , little pani formed in bulk solution ( away from the surface of ncss ), as confirmed by sers measurements of remixing freshly dialyzed ncss with the supernatant after separation of the x - ray irradiated aniline - ncss solution via centrifugation . for 0 . 1 mm aniline and 4 - nm ncss , we estimated & lt ; 1 monolayer of aniline on the surface of ncss if aniline is uniformly deposited on ncss , which partially explains the observed results . fig1 b shows a typical sers spectrum ( solid black line ) of pani from 0 . 1 mm aniline in dialyzed ncss aqueous solution irradiated with 25 - gy x - rays at 2 . 5 gy / min dose rate . this dose rate was used throughout this work unless otherwise noted . also shown is a sers spectrum of pani formed spontaneously from monomers mixed with dialyzed ncss without x - ray irradiation ( dashed grey line ). the intensity of sers signal detected here was used to quantitatively assess the amount of pani , and the enhancement of pani formation on ncss due to x - ray irradiation is & gt ; 10 times using the ratio of the integrated intensities of peaks of these two samples in the 1285 - 1550 cm − 1 region . although higher enhancements may be obtained using peaks of these two signals at a certain specific raman peak , the integrated method is chosen because it is less biased by potential charge transfers between pani and ncss that may result in peak shift ( billman j et al ., surf sci . 1984 , 138 , 1 - 25 ). the rate of pani formation from aniline in water under x - ray irradiation without ncss was much lower , for it required 10 mm monomers and 125 gy to produce pani of the same sers signal strength as from 0 . 1 mm aniline and 25 - gy x - rays with ncss . if first - order rate laws apply here , then ncs under x - rays increased the yield by 400 times over x - ray irradiated aniline aqueous solution without ncss using the signal integrated in the 1285 to 1550 cm − 1 region . the enhancement was found to be a function of x - ray ( 100 kvp ) dose rate : at 0 . 6 gy / min the enhancement was 28 . 0 times ( data not shown ). the enhancement only changed mildly , less than 20 %, as the x - ray tube voltage was varied between 50 and 100 kvp and with the use of a tin foil ( 0 . 25 mm thick ) to modify the x - ray spectrum . future experiments using monochromatic x - ray sources at synchrotrons may be necessary to completely unravel the subtle dependency of enhancement on x - ray energy . fig1 c shows sers spectra of pani formed from as - synthesized ncss in 0 . 1 mm aniline aqueous solution , both with ( solid black line ) and without ( dashed grey line ) x - ray irradiation . the sers pattern is different from the dialyzed ncss . the enhancement is lower , at approximately 6 times . to prove that sers can be used to measure the amount of pani and to quantitatively estimate the enhancement , fig1 d shows the peak intensity of pani sers in the 1285 to 1550 cm − 1 using dialyzed ncss under x - ray irradiation as a function of x - ray dose , and approximate linearity is observed within the specified dose range . this linearity and a similar linearity observed between sers signal intensity and pani concentration in solution ( for sers signal intensity up to 10 cps as shown in fig2 ) suggest that sers probe can be used to quantitatively estimate the amount of pani and hence enhancement . two forms of pani , emeraldine salt ( es ) and emeraldine base ( eb ) made in solution were examined with sers ( spectra shown in fig3 a ) ( huang j x et al ., j . am . chem . soc . 2004 , 126 , 851 - 855 ). the ratio of peaks at 1417 and 1451 cm − 1 was used to determine the type of pani . based on sers profiles and using the spectra obtained by baibarac et al . as a reference , pani formed on dialyzed ncss was determined to be eb on au substrate ( baibarac m et al ., synthetic met . 1998 , 96 , 63 - 70 ). sers spectrum of pani made in water under x - ray irradiation without ncss was also obtained ( fig3 b ). dialysis removes free ions and other compounds in ncss after synthesis , but it has been reported that ag ionic species may slowly depart ag nanoparticles in solution over time ( kittler s et al ., chem . mat . 2010 , 22 , 4548 - 4554 ). however , immediately after dialysis the solution had a limited amount of ionic species , as verified by stem inspections , the amount of pani , and atomic absorption ( aa ) measurements . aa measurements showed free ag and au ions and small nanoparticles in dialyzed ncss between 0 . 1 ( detection limit ) to 0 . 25 ppm au or ag in the supernatant after centrifugation separation of dialyzed ncss . taking into consideration that 10 min of 5000 rpm centrifugation could not pull down small nanoparticles ( e . g ., & lt ; 2 nm ) in water , the amount of free ions in the dialyzed ncs solutions should be less than the measured 0 . 1 ppm au or 0 . 25 ppm ag ions , which correspond to 0 . 6 μm au ions or 2 . 3 μm ag ions . to further investigate the formation mechanisms and the origin of the unique sers profile for dialyzed ncss , we tested pani formation in solutions of ag and au ions mixed with aniline and dialyzed ncss . base on ncss sers measurements , no additional pani formation with up to 1 μm au and ag ions added to aniline and irradiated with 25 - gy x - rays . without x - ray irradiation , 1 mm ag + and 1 mm aniline ( fig4 a ) and 10 μm of au 3 + and 0 . 1 mm aniline ( fig4 b ) ( both incubated for 18 hours ) yielded a small amount of pani similar to that produced with dialyzed ncss and 0 . 1 mm shown in fig1 b ( dashed grey line ), proving free ions along cannot cause pani formation . au / ag in ncss may be oxidized by ros species produced from x - rays irradiation of water . typically 2 . 7 μm accumulated oh radicals (. oh ) are generated after a 10 - gy x - ray exposure , although instant ros concentrations are extremely low (˜ pm ) ( el omar a k et al ., j . phys . chem . a . 2011 , 115 , 12212 - 12216 ). hence it is possible to assume that individual ros react with ncss at low x - ray dose rate (˜ 1 gy / min ). fig5 a shows a tem image of ncss irradiated with 450 - gy x - rays without aniline . the ag cores were clearly etched away by radiation , and sharp au edges were smoothened , which can only be caused by oxidative . oh reacting with ncss . the release of ag ions are similar to what have been reported in the literature using . oh smoothing rough au surfaces ( nowicka a m et al ., angew . chem . int . edit . 2010 , 49 , 1061 - 1063 ). in the presence of x - rays or monomers , these ions may be reduced to form nanoparticles . indeed , small nanoparticles were detected even after low - dose irradiation of ncss in the presence of aniline , as shown in fig5 b , which is a high resolution stem image of ncss irradiated with 25 - gy x - rays in 0 . 1 mm aniline . the contrast of the image was increased to show the small nanoparticles around ncss . testing was done by adding . oh scavengers to stop the growth of pani and the release of ions from ncss . dimethylsulfoxide ( dmso ), an . oh scavenger , was used and pani formation was significantly reduced to below the sers detection limit . fig5 c shows an stem dark - field image of ncss in monomers with 10 mm dmso under 25 - gy x - ray irradiation , showing no small nanoparticles . this confirmed that . oh was responsible for the creation of the species that promoted polymer formation on ncs surface . to test the role of the surface of nanoparticles , silica nanoparticles were employed and no pani formation was detected after 25 - gy x - ray irradiation . these results suggest that pani growth occurs on the surface of ncss during x - ray irradiation , and little polymers are grown in bulk solution at low aniline concentrations (≦ 1 mm ). to further test the role of surface and the role of ag ionic species , micelles were employed to isolate the contact between sers substrate and monomers . fig6 a shows the schematic of the experiment , and fig6 b shows the result of using micelles and a chemical oxidizer to help form pani in micelles without x - rays . if pani formation does not rely on the surface , but merely ag ionic species , then pani should form when ncss are irradiated with x - rays because it results in ag ions leaving ncss . in the case of micelle / ncss irradiated with x - rays , growth stopped , suggesting that the surface of ncss , in addition to ionic species , is a critical component to the growth of polymers . these observed enhancements are unlikely caused by enhanced x - ray absorption by ncss for the following reasons . first , theoretical calculations using a package created in this lab show that direct absorption of x - ray by ncss at the concentrations used here (˜ 4 nm ) is less than 7 % of that by the surrounding water in which ros are generated . this means that type 1 physical enhancement ( t1pe ) or average enhancement is negligible ( lee c et al ., j . phys . chem . c 2012 , 116 , 11292 - 11297 ; cheng n n et al ., j . am . chem . soc . commun . 2012 , 134 , 1950 - 1953 ). even considering the nanoscale energy deposition enhancement or type 2 physical enhancement ( t2pe ), the amount of enhancement over water within a nanoscale volume is less than 9 times for the size of 60 nm ncss , which is much less than the observed 400 - time enhancement observed here ( lee c et al ., j . phys . chem . c 2012 , 116 , 11292 - 11297 ). secondly , the sers pattern for pani grown from ncs s / aniline solution is different from that of pani produced from aniline under irradiation and then probed with ncss , as shown in fig1 b and 3b . third , adding dmso almost completely stopped pani formation ; proving that the . oh reacting with aunps are not generated locally from ncss but from surrounding water . lastly , the enhancement is a function of dose rate , which cannot be readily explained with x - ray absorption enhancement . as a result , the observed enhancement was considered to be mainly caused by some type of chemical enhancement similar to that reported by cheng et al . ( cheng n n et al ., j . am . chem . soc . commun . 2012 , 134 , 1950 - 1953 ). fig7 shows a schematic of proposed chemical enhancement processes . without wishing to be bound by theory , it is thought that ros such as . oh created by x - ray irradiation of water help oxidize ncss , creating positive charges on the surface , first on au as it dominates the surface ( step 1 in fig7 ). these positive charges then migrate to ag sites due to the galvanic effect ( step 2 ). without monomers , ag ions may leave ncss as shown in fig5 a ; with monomers , especially when there are monomers are already physically adsorbed on the surface near the hot spots for sers , both au and ag cationic species can promote polymer formation ( step 3 ). this is a new type of chemical enhancement of nanostructures enhancing the effect of x - rays , taking advantage of the scavenging property of these ncss toward . oh and the galvanic process between au and ag in ncss . although a . oh can directly react with an aniline molecule deposited on ncss , it is much easier that the whole ncs gets oxidized by the . oh to enable polymerization . the proposed sequence shown in fig7 also favors low dose rate of x - rays so that . oh can react with ncss without the interference from reducing ros such as solvated electrons ( e − aq ) and superoxides . at higher dose rates , oxidation and reduction are more likely to happen simultaneously , working against each other and canceling the effect . this is clearly shown in the dose rate dependency results , showing 0 . 6 gy / min resulting in almost 3 times the enhancement obtained at 2 . 5 gy / min . once formed , pani can only change form , but not be destructed by reducing radicals such as superoxides . the spontaneous pani formation in the presence of ncss can be explained by the residual ionic species ( not free ions ) left after nanomaterial synthesis . such spontaneous pani formation was observed in many forms of nanomaterials with persistent spontaneous polymerizing ability even after dialysis , making it difficult to measure the enhancement due to x - ray irradiation . the dialyzed ncss are the first and best nanostructures that produce minimal spontaneous polymerization while allowing the highest enhancement under irradiation of x - rays . it is worth pointing out that pani sers spectrum may vary if ncss were irradiated with the laser for a long time ( 10 min ) ( data not shown ). however , the sers signals were consistent under mild illuminating conditions and for a short time (& lt ; 3 min ). the sers spectra were obtained by removing a broad background that spans over the whole 1100 - 1700 cm − 1 . this background was observed before , e . g ., by mallick et al . ( mallick k et al ., macromol . rapid comm . 2005 , 26 , 232 - 235 ) and it is dependent of the growth condition . finally , effects of radiation on pani was studied in the past ( wolszczak m et al ., rad . phys . chem . 1996 , 47 , 859 - 867 ), which suggests that the damage to pani to be relatively small for the amount (& lt ; 25 gy ) of x - rays . the results and concepts shown here may find a wide range of applications in the area of x - ray nanochemistry . for instance , this will benefit polymerization - induced drug release at remote locations using highly penetrating x - rays . remote controlled polymerization may also help create new applications in optics and semiconductor industries . using low dose x - rays to release ag ions may have applications in medicine , especially given the fact that ag ions are known antimicrobials ( dair b j et al ., j . nanosci nanotechno 2010 , 10 , 8456 - 8462 ), although a much lower dose ( e . g ., 0 . 01 gy ) of x - rays will be needed to fully take advantage of this process . a 28 - time enhancement of polymerization of aniline on ag / au nanocoreshells ( ncss ) was achieved with x - ray irradiation at 0 . 6 gy / min dose rate . the enhancement of more than 400 times was measured for polymerization of aniline under x - ray irradiation but without ncss . the enhancement was x - ray dose rate dependent and dialyzed ag @ au ncss generated higher enhancement . we confirmed that sers can be a qualitative method to determine the amount of molecules on the sers substrate . the proposed mechanism is that x - rays first generate reactive oxidative species ( ros ) that react with ncss to form au or ag ionic species embedded within ncss . this ionic complex then oxidizes aniline already absorbed on ncss to form pani . reagents were purchased from sigma and used without further purification . ag @ au ncss were synthesized following a procedure by kumar et . al . and purified with dialysis ( kumar g v p et al ., j . phys . chem . c . 2007 , 111 , 4388 - 4392 ). briefly , silver nanoparticles ( agnps ) were first synthesized using a 1 wt . % trisodium citrate reduction of 0 . 529 mm agno 3 at reflux . to form the gold shell , 6 . 25 mm hydroxylamine and 0 . 465 mm chloroauric acid were slowly added simultaneously in a dropwise fashion to a vigorously stirring solution of 12 . 5 ml agnps in 10 ml water ; after addition , stirring was reduced and continues for 15 minutes . the ncs solution was transferred to a polypropylene falcon tube and allowed to rest overnight . a 15 ml portion of the ncs solution was then dialyzed using a 10k mwco dialysis cassette ( thermo , slide - a - lyzer ). the dialysis bath was refreshed after the first and third hours during total dialysis time of 18 hours . both the as - synthesized and dialyzed solution was used in a 9 : 1 ratio with aniline monomers for radiation experiments . radiation experiments were carried out in a home - built irradiation chamber housing a 65 w microfocus x - ray source ( thermo kevex , pxs10 - wb - 10 mm ). samples were irradiated individually at 10 - 500 μa and a fixed 100 kvp . the optimum dose rate used is 50 μa which corresponded to 2 . 5 gy / min to the sample volume . sers was performed on a home - built raman microscope using a 780 - nm diode laser ( l47855 - 95 - te , micro laser systems ). the raman signal was collected using a 63 × microscope objective and an ultra - steep long pass filter ( iridian ). the signal was collected via an optical fiber to a spectrometer ( spectrapro 300i , acton research corp ) with a liquid nitrogen cooled ccd ( spec - 10 , roper scientific ). the raman shift was calibrated against the well - known sers spectrum of p - nitrothiophenol . spectra were acquired at the laser power of 90 mw and an acquisition time of 3 minutes . the raw spectra of pani growth possessed a large broad background which was removed using a spline function . radiation samples were prepared by combining ncs solutions with aniline monomers in a 9 : 1 ratio so that the concentration of aniline monomers is between 0 . 01 mm and 1 mm for radiation experiments . the samples were then irradiated individually . before and after irradiation , a 25 μl aliquot was taken and aggregated by spiking 1 μl of 1 m na 2 so 4 solution . sers measurements were performed on the droplet of sample on a glass cover slip . the sample without irradiation was allowed to incubate with monomers for 10 minutes , the length of time equivalent to a 25 - gy exposure . water and metal ion samples were prepared in a similar 9 : 1 ratio with monomers , but using milliq water or 10 − 7 - 10 − 3 m ag + or au 3 + in place of the ncs solution . those samples were combined with ncss for sers after allowing monomers and ions to incubate overnight and similarly aggregated prior to acquiring sers spectra . when investigating metal ions in addition to ncss , metal ions and monomers were added to ncs immediately and then allowed to incubate for 10 minutes . in the case of aniline micelles , a mixture of 25 - mm aniline monomers and 12 . 5 - mm sodium dodecyl sulfate ( sds ) was used . the micelle control was chemically oxidized using sodium peroxidisulfate to form pani - micelles according to peng et al . ( peng z q et al ., langmuir 2006 , 22 , 10915 - 10918 ). radiation experiments were performed identical to that of aniline monomers shown above . additionally , emeraldine salt and emeraldine base pani nanofibers were synthesized chemically for use as standards following a method by huang et al . ( huang j x et al ., j . am . chem . soc . 2004 , 126 , 851 - 855 ). atomic absorption ( varian , spectraa 220fs ) measurements were performed on the supernatant of a sample after centrifuging at 5000 rpm for 10 minutes to remove ncss but retain small (& lt ; 2 nm ) nps and metal ions . transmission electron microscopy ( tem ) and scanning tem ( stem ) stem samples were prepared by a drop - dry method . samples were centrifuged and the soft pellets resuspended in 100 % ethanol to reduce drying time and increase uniformity . tem image ( fig5 a ) was acquired on a jeol 1230 and stem images ( fig1 a , 5b , 5c ) images were acquired on a jeol 2100 . fig1 a and 5a are optimized contrast from the instrument computer , whereas fig5 b and 5c have undergone brightness and contrast adjustments post imaging to emphasis presence or absence of small nps near the larger ncss . 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