Patent Application: US-74158608-A

Abstract:
the invention relates to a multimetallic nanoshell sensor which comprises a core that is less conductive that a first metallic layer and having a catalytically active second metallic layer partially or completely surrounding the first metallic layer . the sensor can be used in any surface enhanced spectroscopic applications .

Description:
as a demonstration of the invention , palladium ( pd ) was deposited onto gold ( au ) nanoshells , to form pd - on - au nanoshells ( pd / au ns &# 39 ; s ). these nanoshells were assembled onto a flat film , and the nanoshell ensemble was used to study the catalysis of trichloroethene hydrodechlorination ( tce hdc ), an important chemical reaction for groundwater treatment , as an exemplary system to demonstrate the increased sensitivity contributed by the second metallic layer . it was found that pd / au nanoshells were much more sensitive to low concentrations of dichloroethene ( related to tce ) than plain au nanoshells were . the advantages of multimetallic nanoshell - sers over other techniques are that water does not interfere with the spectroscopy and that the chain of surface reactions can be observed while it is happening . the pd / au nanoshells described herein have reactive pd “ islands ” on the shell surface , that increase the sensitivity of regular , uncoated nanoshells for certain chlorinated chemical compounds . pd / au nanoshells are demonstrated herein to be much more sensitive than plain au nanoshells in detecting these chlorinated compounds . other coating metals can be chosen that are reactive for other compounds . further , the surface - bound compounds can be induced to react with other compounds , and this surface reaction ( or surface reactions ) can also be observed . the multi - step preparation is detailed as follows ( fig1 a ). au nanoshells ( nss ) were prepared as described previously . briefly , 1 ml of 120 - nm diameter silica colloid ( precision colloid p120 ) was added to 26 mm aminopropyltrimethoxysilane in ethanol , and aged overnight . the solution ( 500 μl ) was then added to 40 ml of a sol of 1 - 2 nm au nps , previously prepared via the duff method and aged for at least 2 weeks . the sample was aged overnight to allow the electrostatic self - assembly of the negatively charged au nps to the positively charged amine groups on the modified silica colloid . the sample was then centrifuged and redispersed in deionized water three times to ensure the removal of excess au nps , leaving a final volume of ˜ 1 ml . the sample ( 15 μl ) was added to 3 ml an aqueous solution of 370 μm haucl 4 and 18 mm k 2 co 3 and stirred . the gold salt was then reduced onto the silica cores by adding 30 μl of 30 wt % formaldehyde and shaking vigorously for 5 minutes . the sample was then centrifuged and redispersed in deionized water to ensure the removal of excess gold salt . final volume of the solution was 3 ml . the uv - visible spectrum of the au nss was fit using mie scattering theory to determine the size and concentration of particles . quantitative agreement between experimental and theoretical ns spectra was obtained for 60 - nm radius sio 2 core and 22 - nm au shell thickness ( fig2 a , b ), with the overall radius of 82 nm confirmed by analyzing 200 particles with sem ( figure s 1 ). the concentration of nss was determined to be 1 . 18 × 10 8 particles / ml . au nss coated with a sub - monolayer of pd metal ( pd / au nss ) were synthesized analogously to the previously reported synthesis of pd / au nps . h 2 pdcl 4 ( 7 . 2 μl , of 2 . 4 mm solution ) was added to 18 ml of the as - synthesized au nss and stirred . the pd salt was reduced by bubbling the solution with uhp h 2 gas for 5 minutes . scanning electron microscope ( sem ) images and electron dispersive x - ray ( edx ) spectra of the pd / au nss were obtained using a fei quanta 400 . for particle size analysis , 30 μl of either au nss or pd / au nss was drop - dried onto an sem stub . for edx analysis , pd / au nss were repeatedly dropdried onto a stub until a visibly thick layer was formed . the spectrum was obtained using a edx detector attached to the sem . in order to characterize any changes in plasmonic behavior upon the addition of pd , uv - vis absorbance spectra of the nss were collected on a shimadzu uv - 2401 pc spectrophotometer using a polystyrene cuvette with a 1 - cm path length . the average pd / au ns diameter was found to be 164 nm , according to sem analysis ( fig1 c ), indicating no detectable size increase after the addition of pd metal . electron dispersive x - ray ( edx ) spectroscopy confirmed the presence of pd ( fig . s 1 ). we estimated that the au ns surfaces had a ˜ 10 % coverage of pd atoms ( meaning ˜ 10 % of complete pd monolayer coverage ). the final composition of the pd - decorated au nss was estimated at 6 . 7 wt % sio 2 , 90 . 6 wt % au , and 2 . 7 wt % pd . the plasmon resonance spectrum of the au nss decreased in intensity and red - shifted slightly with the addition of the pd metal , similar to what had been observed with pd - coated au nanoparticles of much smaller size . the relatively large imaginary part of the pd dielectric constant in the visible light regime damps the plasmon resonance , as seen in the mie theoretical modeling of au nanoshells with 100 % pd coverage ( fig2 b ). to immobilize the nss , 5 - mm × 5 - mm × 0 . 5 - mm polished si wafers were cleaned in 3 : 7 solution of 30 wt % h 2 o 2 and 30 wt % h 2 so 4 for 1 h . the wafers were then rinsed in deionized water and dried , before adding to a solution of 0 . 1 wt % poly ( vinylpyridine ) ( mw ˜ 40 , 000 ) in ethanol . after aging overnight , the wafers were rinsed in ethanol and dried . 50 μl of a concentrated solution of the nss ( volume reduced from 18 ml to 128 μl by centrifugation and removal of supernatant ) was then pipetted onto a wafer and allowed to assemble overnight , schematically shown in fig1 b . after rinsing with deionized water to remove unattached nss , the wafer was adhered to the center of an 18 mm round microscope coverslip using slide adhesive . samples used in the reaction and chemisorption experiments were then plasma - cleaned under vacuum ( harrick plasma cleaner / sterilizer , pdc - 32g ) to remove surface impurities for 2 min just prior to raman analysis . to determine the sers effectiveness of pd / au nss relative to the au nss , 10 μl of 440 μm paramercaptoaniline ( pma ) in ethanol was added to samples prepared as in the previous section . pma is a highly raman active species , and has been used in a number of experiments to determine sers efficiency . the samples were aged overnight to ensure the formation of a complete monolayer on the nanoshell surfaces in parafilm - sealed petri dishes , and then rinsed with ethanol prior to analysis . sers spectra were obtained using a renishaw in via micro - raman spectrometer with a 785 - nm excitation laser and a 40 × working distance objective . spectra were obtained using 0 . 05 mw power and 10 second integration times . for each sample , eight spectra were acquired at different spots and averaged . standard deviation between spectra for each sample was less than 5 %. to perform the adsorption ( or chemisorption ) and reaction experiments , the plasma - cleaned sample was mounted in a sealed analysis chamber with inlet and outlet ports ( warner instruments rc - 43 , 213 μl volume without pd / au ns sample ), and mounted inside the raman spectrometer ( figure s 2 ). prior to the acquisition of spectra from the substrate , the reduction of pd was ensured by flowing 10 ml of h 2 saturated deionized h 2 o and allowing to sit for 10 minutes . we chose to study 1 , 1 - dce ( 1 , 1 - dichloroethene ), a less chlorinated form of tce that lends itself to simpler spectral interpretations and that is hypothesized to be a reaction intermediate for tce hdc . solvents used to perform the chemisorption and reaction experiments were prepared by adding 180 ml of deionized water each to boston round screw top bottles ( alltech , 250 ml ). the threads were wrapped with teflon tape and sealed with a teflon - rubber septum . two bottles ( one used for catalyst rereduction and the other , for rinsing ) were bubbled with uhp h 2 and n 2 , respectively , for 1 hr . additional bottles were bubbled for 1 hr with n 2 or with h 2 : n 2 gas mixtures ( volume ratios of 20 : 80 and 100 : 0 ) for use in the chemisorption and / or reaction experiments . the h 2 concentrations present in the aqueous phase were estimated to be 16 . 3 and 81 . 9 mm , respectively , using a henry &# 39 ; s law constant of 1228 atm / m . after the bubbling step , 1 or 5 μl of 1 , 1 - dce was added , such that the h 2 : 1 , 1 - dce molar ratio was the same ; the amount of 1 , 1 - dce dissolved in the liquid phase was estimated to be 50 . 9 μm (= 4 . 9 ppm ) or 254 . 5 μm , respectively , using a henry &# 39 ; s law constant of 26 . 1 atm / m and a density of 1 . 21 g / l . the liquid - phase h 2 amount was set approximately 6 % in excess to what is needed for complete conversion of 1 , 1 - dce to ethane ( ch 2 = ccl 2 + 3h 2 → ch 3 ch 3 + 2hcl ). the bottles were placed on a rocking platform to allow dissolution and equilibration of 1 , 1 - dce between the gas and liquid phases . spectra ranging from 100 - 1700 cm − 1 raman shifts were obtained using 0 . 76 mw of laser power and 10 second integration times , a total acquisition time of one minute per spectrum . ten spectra were obtained at a single spot prior to the changing of solvents to establish a baseline and ensure the cleanliness of the substrate . for the chemisorption experiments , the analysis chamber was first flushed with n 2 - saturated h 2 o to remove excess h 2 from the cell , and to remove possible chemisorbed h 2 . the chamber was then flushed with 3 ml of the n2 - saturated 1 , 1 - dce solution ( 50 . 9 or 254 . 5 μm ) and spectra were collected over time . similar experiments were performed using au nss ( without pd metal ). experiments involving a chemisorption - reaction sequence were performed similarly . after flushing with n2 - saturated h 2 o , 3 ml of the 254 . 5 μm sample was added . spectra were obtained repeatedly for 30 minutes , after which 1 ml of either 0 : 100 , 25 : 75 , or 30 : 70 h 2 : n 2 was quickly flushed through the cell , and spectra were taken continuously until no other changes occurred . for the reaction experiments , 3 ml of low - concentration ( 1 , 1 - dce , 50 . 9 μm ; h 2 , 16 . 3 mm ) or high concentration ( 1 , 1 - dce , 254 . 5 μm ; h 2 , 81 . 9 mm ) solutions were added , and spectra were collected over time until no noticeable changes in the spectra were observed . baseline corrections in the spectra were performed by normalizing to the silicon 520 cm − 1 mode , to account for any minor drifts in focus over the experimental time frame . the baseline was corrected by subtracting an average of the initial ten spectra taken to determine surface cleanliness . to analyze the products using the sers analysis chamber , the same protocol was used as in the sers reaction experiments , except that aliquots of fluid were removed (˜ 200 μl ) at 0 , 12 , 30 , 41 , or 100 minutes with a 1 - ml needled syringe and injected into a 2 - ml septum - capped vial . after allowing the gas and liquid phases of the sample to equilibrate for 30 min , 250 μl of headspace gas was withdrawn with a gas - tight syringe and injected into an agilent technologies 6890 gc equipped with a flame ionization detector ( fid ) and a packed column ( 6 - in × ⅛ - in outer diameter ) containing 60 / 80 carbopack b / 1 % sp - 1000 ( supelco ). calibration curves were prepared for chlorinated ethenes , chlorinated ethanes , ethane , and butane . the zero time point samples were verified to match the initial 1 , 1 - dce concentrations of 50 . 9 and 254 . 5 μm for the low and high concentration experiments , respectively . upon analysis of the products , it became apparent that , at later time points , the carbon balance did not always close , possibly due to evaporation from the gas - tight syringe before injection into the gc . to correct for this , we assumed evaporation of all components was equal and re - scaled the measured concentrations with the same correction factor , such that the total carbon balance was met for all time points . the superficial first - order rate constant was determined by linear fitting of ln ( c / c 0 ) versus time profiles , where c is 1 , 1 - dce concentration and c 0 is the initial 1 , 1 - dce concentration . selectivities were calculated by dividing the concentration of each product by the amount of 1 , 1 - dce reacted . the intensity of the sers spectrum obtained from pma - functionalized au and pd / au nss decreased with the presence of pd on the ns surface due to damping of the au ns plasmon resonance ( fig2 c ). an interesting feature of this sers spectrum is the shift in the intense , low - frequency peak near 390 cm − 1 ( fig2 c , inset ). this vibrational mode was assigned to coupling between the metal - sulfur bond stretch and a pma ring deformation . this peak shifted to 406 cm − 1 with the addition of pd , indicating an increase in the surface - pma bond strength and the possible binding of pma to pd surface atoms . fig2 d shows the sers spectra for 1 , 1 - dce over au and pd / au nss . in contrast to the pma case , it is readily apparent that pd increased the 1 , 1 - dce band intensities despite the damping effect of pd on au ns extinction . fig3 a - c shows the results of contacting the pd / au nss with an aqueous solution of 1 , 1 - dce at 50 . 9 μm . after an induction period , there appeared to be two different time - dependent adsorption states : the initial state ( 0 - 20 minutes ; fig3 b ), with raman spectra featuring bands at 214 , 954 , ˜ 1060 , ˜ 1160 , ˜ 1250 , 1430 , and ˜ 1550 cm − 1 , and the final state ( 37 - 52 minutes ; fig3 c ), with bands found at ˜ 225 , ˜ 390 , ˜ 1165 , ˜ 1455 , and ˜ 1500 cm − 1 . the peaks can be assigned to wavenumber regions that represent particular raman - active vibrational modes of surface - bound 1 , 1 - dce , based on reported assignments for chemisorbed ethylene 34 , tce 35 , free 1 , 1 - dce , and corroborated with ab initio density functional theory calculations . peaks in the 1500 - 1600 cm − 1 , 1220 - 1290 cm − 1 , and 1000 - 1100 cm − 1 range were assigned to cc stretching , ch 2 scissoring , and ch 2 wagging modes of π - bound 1 , 1 - dce , respectively . the sharp peaks at 954 , ˜ 1160 , and 1430 cm − 1 were further assigned to the cc stretching , ch 2 wagging , and ch 2 scissoring modes of di - σ - bound 1 , 1 - dce . the other closely located peaks suggest different metal adsorption sites , sites of different binding strengths , or binding states intermediate to those of the π - and di - σ - bound 1 , 1 - dce species . the low - frequency features below 400 cm − 1 may be due to c - m ( m = metal ) or c — cl bonds . these results provide direct evidence of 1 , 1 - dce undergoing chemisorption from water . 1 , 1 - dce adsorption is dynamic , as the initial state changes substantially into a new stable state ( fig3 a ). signals for π - bound 1 , 1 - dce were lost , with residual di - σ - bound 1 , 1 - dce showing the weak peaks at ˜ 1165 and ˜ 1455 cm − 1 . a sharp and intense peak at ˜ 1500 cm − 1 may be due to the cc stretch of vinylidene (═ c ═ ch 2 ) species ; this value is slightly blue - shifted from the theoretical value of 1490 cm − 1 for vinylidene on pd ( 111 ) 37 and red - shifted from that for vinylidene on si surfaces 38 , and in the range predicted by our ab initio calculations . the bands at 230 cm − 1 and ˜ 390 cm − 1 can be attributed to cl - m and c — m bond stretchings , respectively , consistent with the removal of chlorine from 1 , 1 - dce to form vinylidene . this process is summarized in scheme 1 . fig3 d - f shows the results of contacting the pd / au nss with an aqueous solution of 1 , 1 - dce at a higher concentration of 254 . 5 μm . unlike the 50 . 9 μm case , raman peaks appeared almost immediately after injection of the dce solution . the peaks at ˜ 220 and , ˜ 400 cm − 1 ( cl - m and c - m bond stretchings ) indicated dce dechlorination . bands centered at ˜ 1500 cm − 1 and ˜ 1200 cm − 1 were quite broad , spanning at least 100 cm − 1 , suggesting vinylidene and other adsorbed species . the band positions did not change much with time , but their intensities increased continuously until ˜ 20 minutes , after which the spectra stabilized . it is likely that the broad band at ˜ 1500 cm − 1 represents unsaturated oligomeric species on the metal surface , as they have previously been assigned to conjugated olefins . this is not unexpected , as previous kinetic studies with tce using pd - based and pd / au - based catalysts have reported trace amounts of carbon - coupling products at low h 2 / tce concentration ratios . these olefinic species were observed due to the higher dce surface concentration , leading to increased interactions between chemisorbed dce species ( i . e ., surface crowding effect ). these species could be removed from the ns surface by contacting with h 2 - containing water ( fig . s 3 ) but not with n 2 - containing water ( fig . s 4 ). control experiments using au - only nss and 50 . 9 μm ( fig2 d ) and 254 . 5 μm dce ( fig . s 5 ) solutions showed no raman peaks at all . these results indicated that solvated 1 , 1 - dce could not be detected at these concentrations and was observable only in the presence of pd metal , suggesting pd ensembles or pd — au mixed sites as active sites for chemisorption . while the surface structure of pd on the au nss is not known precisely , the metal mostly likely are present as two - dimensional atomic ensembles or islands . this study shows the successful synthesis and application of pd - supported au nss for the detection of water - phase adsorbates . the pd metal provided direct binding sites on the au surface ( either as pd ensembles or pd — au mixed sites ), effectively lowering the concentration detection limit of 1 , 1 - dce for sers to at least 4 . 9 ppm and extrapoloation of fig2 d suggests that detection could easily go as low as 490 ppb ( 10 times lower ). significantly , chemical reactions of adsorbate species can be observed as they proceed on the catalyst surface with time , providing a newfound ability to detect and identify reaction intermediates in water under ambient conditions in situ . with further development in improved time and raman peak resolutions under steady - state flow conditions , ns - enabled sers may lead to new mechanistic insights into other liquid - phase chemical reactions , like gold - catalyzed glycerol oxidation and platinum catalyzed electroreduction of oxygen , for which spectroscopic analysis is lacking . to date the invention has been exemplified with a gold first layer and palladium second layer . however , nanoshells of other metals have already been successfully used in sers and other surface enhanced spectroscopic techniques . thus , silver , copper and first metallic layers can also be used , as these metals have already proven to be useful . further , any platinum group metal can be used in the second layer as what is required is a catalytically active second metal that can attract and possibly react with the chemical to be detected and this group of metals have proven catalytic activity in a wide range of reactions . thus , iridium , osmium , palladium , platinum , rhodium , and ruthenium are all expected to be useful in the invention . to test this hypothesis , cores can be coated with gold silver or copper and each can then be coated with iridium , osmium , palladium , platinum , rhodium , and ruthenium , thus preparing a set of 18 multimetallic nanoshells , each of which can be tested for sensitivity in detecting an appropriate chemical , as described in the prior examples . 1 . a . tolia , t . wilke , m . j . weaver , c . g . takoudis , chemical engineering science 47 , 2781 ( june - 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