dendrimer - encapsulated nanoparticles ( dens ) 
 are well - defined nanoparticles 
 having sizes ranging from just a few atoms to perhaps 300 atoms .
this is the most scientifically 
 interesting range of metal particle sizes because the addition of 
 just a few atoms can drastically change their optical , electrical , 
 mechanical , and catalytic properties . for fundamental studies of catalytic properties , 
 dens are particularly useful for two reasons .
first , it is possible 
 to control their size , composition , and structure over a fairly broad 
 parameter space , which is important for comparing theoretical calculations 
 with experimental data .
second , the presence of the dendrimer protects the 
 particles from agglomeration without poisoning the metal surface . 
 for both of these reasons ,
dens are one of the best model materials 
 available for studying the fundamental properties of electrocatalytic 
 reactions on metal particles in the 12 nm size range .
pt is one of the most important catalytic metals , and hence pt 
 dens have been studied as catalysts for homogeneous , heterogeneous , and electrocatalytic reactions .
however , we and others have previously pointed out that correlations between theory and 
 experiment with dens are complicated by incomplete reduction of the 
 pt salt used as the nanoparticle precursor .
this situation is unique 
 to pt dens and is a consequence of the method used to prepare them . 

first , the poly(amidoamine ) ( pamam ) dendrimer and precursor metal 
 salt are mixed together , and this results in encapsulation of the 
 precursor within the dendrimer interior .
second , a strong reducing 
 agent like bh4 is added to the resulting 
 solution .
this leads to reduction of the precursor and subsequent 
 intradendrimer agglomeration of the resulting atoms to yield the final 
 nanoparticle . for most metals , the addition of bh4 results in complete reduction of the precursor 
 metal salt .
pt is 
 unusual , however , in that the synthesis leads to a bimodal distribution 
 of fully reduced dens and fully unreduced , pt - containing 
 dendrimers .
we explained this observation 
 by invoking a nucleation and growth mechanism for pt dens . within 
 this framework ,
zerovalent pt seeds form in some dendrimers but not 
 in others . in the presence of seeds , additional reduction of pt within that dendrimer
however , if no seed 
 forms , then the metal salt is kinetically trapped in its oxidized 
 form . at this point
we do not know with certainty why seeds form in 
 some dendrimers and not in others , but the problem has been studied 
 by others . for example , borodko 
 et al .
reported that multidentate binding 
 of pt to amine groups within the dendrimer 
 hinders the reduction of the precursor complex to zerovalent particles , 
 presumably by shifting the redox potential of pt to more negative potentials .
subsequently , this same group 
 showed that uv irradiation of the precursor can yield linear pt chains 
 containing 28 atoms , and that these seeds lead to the formation 
 of nanocrystals .
we have shown 
 that small ( 1.5 nm ) , fully reduced pt dens 
 can be prepared using the method of galvanic exchange .
these materials are prepared by synthesizing 
 cu dens using the usual bh4 reduction 
 method , and then a pt salt , such as ptcl4 , is added to the solution . because of their relative reduction potentials , 
 the cu dens are oxidized to cu , and the pt salt is reduced 
 to zerovalent pt .
although this method 
 is highly effective , the largest pt dens that can be formed in sixth - generation 
 pamam dendrimers by galvanic exchange contain just 64 atoms .
this 
 is a consequence of the fact that the maximum number of cu ions that can be sequestered within the dendrimer is 64 and that 
 the cu : pt galvanic exchange stoichiometry is 1:1 .
clearly , it would be advantageous to use this same approach 
 to prepare larger pt dens so that a broader size range of materials 
 could be reliably synthesized . in the present report
, we use 
 more advanced characterization tools , 
 primarily scanning transmission electron microscopy ( stem ) and x - ray 
 absorption near edge structure ( xanes ) spectroscopy , to confirm that 
 the standard bh4-reduction method leads 
 to a bimodal distribution of reduced and unreduced pt - dendrimer composite 
 materials .
more importantly , however , we describe a galvanic exchange - based 
 synthetic procedure that leads to larger , fully reduced pt dens .
first , cu dens containing more 
 than 64 atoms are prepared by carrying out multiple sequential complexation / reduction 
 steps .
second , galvanic exchange of these larger cu dens with pt results in pt dens containing up to at least 225 atoms . 

vis 
 spectroscopy , x - ray photoelectron spectroscopy ( xps ) , and extended 
 x - ray absorption fine structure ( exafs ) spectroscopy .
this advance 
 in the synthesis of model pt dens will allow for more accurate comparison 
 of experimental properties to theory , which is the goal of our research 
 in this field .
sixth - generation , hydroxyl - terminated ( g6-oh ) 
 poly(amidoamine ) ( pamam ) dendrimers in methanol were purchased from 
 dendritech ( midland , mi ) . before use
, the dendrimer solution was dried 
 under vacuum and reconstituted in water at a concentration of 100.0 
 m .
isopropyl alcohol and nabh4 were purchased from 
 sigma - aldrich , k2ptcl4 was from acros organics , 
 cuso4 and naoh from fisher scientific , high - purity hclo4 from j.t .
all solutions were made using deionized water having 
 a resistivity of 18.2 mcm ( milli - q gradient system , millipore ) .
the pt - dendrimer precursor complexes ( g6-oh(pt)n , n = 55 , 140 , or 225 ) were 
 prepared by allowing the appropriate amount ( 55 , 140 , or 225 equiv , 
 respectively ) of k2ptcl4 to complex with the 
 interior of g6-oh ( 2.0 m ) for 72 h. the g6-oh(pt)n precursor was reduced by adding 50 
 equiv of nabh4 , and then tightly sealing the container 
 for 24 h. to avoid high h2 pressures , 15 ml samples of 
 dens were reduced in 20 ml vials .
note that in the past we have typically 
 used a nabh4:pt ratio of 10:1 , but it was 
 increased here to facilitate maximum pt reduction .
the first 
 step of the pt den galvanic exchange synthesis is preparation of 55-atom 
 cu dens using a previously reported procedure .
briefly , a 
 2.0 m solution of g6-oh pamam dendrimer was prepared from a 
 100.0 m stock solution . to this
aliquots of 0.30 m naoh 
 were added to adjust the ph to 7.5 , and this solution was 
 stirred for 15 min to allow for cu complexation to the 
 dendrimer . at this point ,
the solution was purged with n2 , and it was kept under n2 for the remainder of the synthetic 
 procedure .
next , an equivalent ( molar ) amount of nabh4 was 
 added from a freshly made 0.10 m stock solution .
once reduction was complete , 
 excess unreacted bh4 was oxidized by 
 adding a 4-fold excess ( relative to the equiv of bh4 ) of 0.1 m hclo4 and allowing it to react 
 for 5 min ( the ph was maintained above 5 with naoh throughout the 
 synthesis ) .
the ph of the solution was then raised to 6.5 
 so that additional equiv of cu could be complexed with 
 the dendrimer .
this cycle ( adjust ph to > 6.5 , add cu , 
 wait for complexation , reduce , oxidize excess bh4 ) was repeated as many times as necessary to synthesize cu dens of 
 the desired size . at this stage ,
the ph was lowered to 3.2 and galvanic 
 exchange for pt was carried out by adding sufficient pt ( from a freshly prepared 0.10 m stock solution of ptcl4 ) to the cu dens solution so that the pt : cu ratio was 1 .
the resulting pt dens solution was immediately 
 immobilized on vulcan carbon using the procedures described in the 
 next two sections .
the g6-oh(pt)n precursor , 
 pt dens synthesized by chemical reduction , and pt dens synthesized 
 by galvanic exchange ( collectively referred to as pt - dendrimer composites ) 
 were immobilized on vulcan carbon by diluting a 2.0 m solution 
 of the pt - dendrimer composite to 200 nm and then adding 2.0 mg of 
 vulcan carbon per ml of diluted pt - dendrimer composite .
this ratio 
 of dendrimers : carbon ensured sufficient separation ( for imaging purposes ) 
 between individual pt - dendrimer composites on the surface of the vulcan 
 carbon .
isopropyl alcohol ( final concentration : 20 vol % ) was added 
 to each solution to assist with dispersion of the carbon .
the resulting 
 ink was sonicated for 1 min , then 2.0 l of this solution was 
 pipetted onto a lacey - carbon - over - ni transmission electron microscopy 
 ( tem ) grid ( electron microscopy sciences , hatfield , pa ) , and finally 
 the grid was dried in air .
a jeol jem - arm200f stem with spherical 
 aberration ( cs ) correction and a high - angle annular dark - field ( haadf ) 
 detector was used for sample analysis . as 
 for the stem analysis , the pt - dendrimer composites were also immobilized 
 on vulcan carbon for xas .
the xas analysis requires a higher surface 
 concentration of the pt - dendrimer composites , however , so 2.0 mg of 
 vulcan carbon per ml of the undiluted ( 2.0 m ) composite was 
 used .
the carbon - supported pt - dendrimer composites were then filtered 
 using an advantec ptfe membrane filter ( 0.5 m pore size ) .
the 
 filtrate was rinsed with water and then isopropyl alcohol , and allowed 
 to dry in air overnight .
the dried powder was pressed into a pellet 
 at 1 ton of pressure for xas analysis .
xas analysis was performed 
 at the national synchrotron light source at brookhaven national lab 
 using beamline x18b .
a pt foil was fit to obtain 
 the amplitude reduction factor ( s0 = 0.87 for the pt l3 edge was used ) .
the first shell was fit in r - space using a k - weight of 2 for the fourier transforms .
a portion of the same 
 pellet used for the xas analysis was used for 
 xps .
the pellet fragment was dissolved in water , and an aliquot was 
 pipetted onto a chip of glassy carbon .
xps was performed using a kratos 
 axis ultra spectrometer ( chestnut ridge , ny ) having an al k 
 source .
individual elemental spectra were collected with a 0.1 ev 
 step size and a band - pass energy of 20 ev .
casaxps 
 ( v 2.3.15 , casa software , teignmouth , uk ) was used for peak fitting , 
 assuming a mixed gaussian / lorentzian model .
before describing 
 our new method for complete reduction of pt dens , we discuss the original 
 bh4-reduction method , which leads to 
 only partial reduction , for comparison .
the uv  vis spectra 
 of the g6-oh(pt)n ( n = 55 , 140 , and 225 ) precursor before and after reduction 
 with bh4 are provided in figure 1 .
these spectra show that even after reduction with 
 50-fold excess bh4 , a fraction of the 
 ligand - to - metal charge - transfer ( lmct ) band of g6-oh(pt)n ( max = 250 nm ) and 
 the absorbance bands arising from the unreduced pt salt 
 ( max = 215 and 230 nm ) are present .
uv  vis spectra of pt - dendrimer complexes , g6-oh(pt)n , and pt dens synthesized by bh4 reduction , g6-oh(ptn ) , where n = 55 , 140 , and 225 . the spectra 
 were acquired using a 2.0 mm quartz cuvette and blanked with 2.0 m 
 g6-oh . as discussed 
 in the experimental section ,
pt dens prepared 
 by galvanic exchange are synthesized by sequentially complexing and 
 reducing aliquots of cu in the presence of g6-oh , and 
 then reacting the resulting cu dens with pt . to prepare 
 cu dens containing more than 64 atoms ,
this is because , as we have previously shown , 
 the maximum number of cu ions that can be complexed with 
 the interior tertiary amines of g6-oh is 64 .
accordingly , the cu den synthesis begins by complexing a slightly 
 substoichiometric amount of cu ( 55 equiv ) with g6-oh . 
 as shown in figure 2a , this results in a well - defined 
 lmct band at max = 300 nm corresponding to the g6-oh(cu)55 precursor . following reduction with bh4
, this lmct band disappears and the characteristic 
 broad absorbance of 55-atom cu dens is observed in figure 2b .
after removal of 
 excess bh4 by addition of hclo4 , 55 additional equiv of cu are added to the solution 
 and the lmct band is observed again ( figure 2a ) , but now it is superimposed on the spectrum of the reduced g6-oh(cu55 ) dens .
importantly , the appearance of the lmct band confirms 
 that no active bh4 is present in solution , 
 because the added cu is still in its oxidized form and 
 able to complex to the dendrimer ( at ph > 6.5 ) .
after the second 
 aliquot 
 of cu is sequestered inside the dendrimer , additional 
 bh4 is added to yield reduced dens containing 
 an average of 110 atoms : g6-oh(cu110 ) .
as shown in figure 2b this results in an increase in the featureless 
 absorbance spanning the indicated wavelength range .
this process is 
 then continued until cu dens of the desired size are formed . in this 
 case
, we stopped the process at 140 total equiv of cu in order to 
 make a direct comparison to bh4-reduced 
 pt dens of the same nominal size ( 140 is a complete - shell magic number 
 for a truncated octahedron nanoparticle ) . (
a ) uv  vis spectra 
 for each sequential complexation step 
 of the synthesis of g6-oh(cu140 ) dens .
( b ) uv  vis 
 spectra for each sequential reduction step in the synthesis of g6-oh(cu140 ) dens .
( c ) comparison of the uv  vis spectra of g6-oh(cu140 ) prepared by direct reduction with bh4 ( blue ) , and g6-oh(pt140 ) prepared by galvanic exchange 
 at ph 3.2 ( black ) and at ph > 6.5 ( green ) .
the concentration of 
 the 
 dendrimers in these solutions was 2.0 m , and the data were 
 obtained using a 2.00 mm quartz cuvette and blanked with water .
the cu dens are converted into 
 pt dens by adding the same number 
 of equiv of ptcl4 as were used to prepare 
 the cu dens . as previously reported for the synthesis of g6-oh(pt55 ) , this galvanic exchange reaction is carried out at ph 3 , with the value here being 3.2 .
figure 2c compares uv  vis spectra of the 140-atom 
 cu dens at ph > 6.5 and the pt dens immediately after galvanic 
 exchange 
 ( ph 3.2 ) .
consistent with previous reports , the absorbance of the 
 g6-oh(pt140 ) dens is significantly higher than that of 
 the corresponding cu dens .
additionally , 
 the cu - lmct band ( max = 300 nm ) is 
 absent after galvanic exchange at ph 3.2 , because the interior tertiary 
 amines of the dendrimer are protonated and hence not available for 
 complexation with cu . for 
 direct comparison with the g6-oh(cu140 ) spectrum , the ph 
 of the g6-oh(pt140 ) solution
was then raised above 6.5 . 
 in this case , the ph is high enough that the interior tertiary amines 
 can complex free cu , and a cu - lmct band is again apparent . 

this confirms that galvanic exchange has occurred and that cu is present in the solution .
pt dens larger than n = 140 can also be made by 
 galvanic exchange .
for example , figure s1 in the supporting information compares g6-oh(pt140 ) and 
 g6-oh(pt225 ) dens .
the absorbance of g6-oh(pt225 ) is higher at all wavelengths , qualitatively indicating the presence 
 of larger nanoparticles .
following synthesis , the pt dens were 
 immobilized on vulcan carbon , 
 which as we have shown previously is a good support for performing 
 electrocatalytic experiments using dens .
immobilization was carried out at ph 3.2 by the addition of vulcan 
 carbon and subsequent filtration as described in the experimental section .
tem images and size - distribution histograms 
 of g6-oh(pt140 ) and g6-oh(pt225 ) immobilized 
 on vulcan , prepared using the aforementioned procedure , are shown 
 in supporting information figure s2 .
consistent 
 with expectations , these data indicate that the dens have diameters 
 of 1.7  0.2 nm for g6-oh(pt140 ) ( calculated diameter 
 = 1.6 nm ) and 1.9  0.2 nm for g6-oh(pt225 ) ( calculated 
 diameter = 1.9 nm ) . the conclusion that a bimodal distribution 
 of fully reduced and unreduced dens results from the standard bh4reduction method relies in part on results 
 from a previous tem study . in that earlier analysis ,
micrographs showed 
 that direct reduction resulted in pt dens having the expected size 
 distribution despite other analytical methods indicating incomplete 
 reduction . at that time , however , we 
 did not have access to electron microscopy with sufficient resolution 
 and contrast to image the g6-oh(pt)n precursor .
now , by utilizing high - resolution aberration - corrected 
 stem , individual pt atoms / ions are visible , and hence it is possible 
 to distinguish between the g6-oh(pt)n precursor and g6-oh(ptn ) dens . 
 the stem data discussed next confirm that bh4 reduction results in a bimodal distribution of reduced and unreduced 
 dens .
figure 3a is a stem micrograph 
 of the g6-oh(pt)55 precursor that has not 
 been exposed to bh4 .
this image shows 
 clusters of individual pt atoms / ions on the vulcan carbon surface . 
 to highlight
the nature of these groupings , red circles having diameters 
 of 7 nm ( the approximate diameter of g6 pamam dendrimers ) have been overlaid onto the image . although 
 highly qualitative , it is not difficult to imagine that these clusters 
 of atoms / ions are contained within individual dendrimers .
the micrograph 
 in figure 3b was obtained after direct bh4 reduction of the g6-oh(pt)55 precursor . in this case , both ordered nanoparticles 
 ( 1.3 nm , indicated by red arrows ) 
 and a grouping of atoms ( red circle ) are visible on the vulcan carbon 
 support .
specifically , the approximate spread of the disordered 
 atoms is maintained at 7 nm , suggesting that some of the complexes 
 are unaffected by the chemical reduction process .
representative stem images 
 of ( a ) the g6-oh(pt)55 precursor complex , 
 ( b ) g6-oh(pt55 ) synthesized 
 by direct reduction with bh4 , ( c ) g6-oh(pt55 ) synthesized by galvanic exchange , ( d ) the g6-oh(pt)140 precursor complex , ( e ) g6-oh(pt140 ) synthesized by direct reduction with bh4 , ( f ) g6-oh(pt140 ) synthesized by galvanic exchange , ( g ) 
 the g6-oh(pt)225 precursor complex , and ( h ) 
 g6-oh(pt225 ) synthesized by direct reduction with bh4 .
the red circles ( 7 nm in diameter ) 
 represent the approximate diameter of a g6 pamam dendrimer , and they 
 highlight the groupings of ions initially present in the precursor 
 complex or after incomplete reduction of the precursor complex .
figure 3c shows that g6-oh(pt55 ) dens synthesized by galvanic exchange of cu for pt reveal 
 no sign 
 of the unreduced g6-oh(pt)55 complex ( that 
 is , no evidence of individual atoms were apparent despite extensive 
 analysis of the grid ) .
this same trend is observed for g6-oh(pt)140 , g6-oh(pt140 ) prepared by bh4 reduction , and g6-oh(pt140 ) prepared by galvanic exchange 
 ( figure 3d , e , f , respectively ) and for g6-oh(pt)225 and g6-oh(pt225 ) prepared by bh4 reduction ( figure 3 g and h , respectively ) . in summary ,
the representative micrographs 
 shown in figure 3 confirm , qualitatively , our 
 earlier bimodal - distribution model , wherein a fraction of the g6-oh(pt)n species remain unreduced when 
 exposed to bh4 , while the remainder 
 are reduced to yield g6-oh(ptn ) dens .
we wish to emphasize , however , that a much larger statistical 
 analysis would be required to confirm these conclusions if they were 
 solely based on electron microscopy . as discussed in the next three 
 sections ,
although it is difficult to obtain quantitative 
 information about the extent of pt den reduction from tem studies , 
 xps is very well suited for this purpose .
accordingly , we used xps 
 to compare the extent of reduction using the bh4 and galvanic exchange approaches .
as shown in figure 4a , the pt 4f7/2 peaks for g6-oh(pt)n ( n = 55 , 140 , and 
 225 ) are present at 72.9 , 73.3 , and 73.5 ev , respectively . these values 
 can be compared with that of the ptcl4 starting material : 73.4 ev ( black vertical line ) .
the slight shift to lower binding energy as the pt : dendrimer 
 ratio decreases may result from the increased availability of dendrimer 
 binding sites at lower pt concentrations and the corresponding 
 increase in multidentate binding .
high - resolution 
 xps spectra of ( a ) g6-oh(pt)n , ( b ) g6-oh(ptn ) synthesized 
 by direct reduction with bh4 , and ( c ) 
 g6-oh(ptn ) synthesized by galvanic exchange 
 ( ge ) , where n = 55 , 140 , and 225 .
the vertical black 
 lines at 73.4 and 71.3 ev represent literature values for the binding 
 energies of ptcl4 and fully reduced 
 pt dens , respectively .
the spectra of the bh4-reduced 
 dens 
 ( g6-oh(ptn ) , n = 55 , 
 140 , and 225 ) exhibit multiple pairs of peaks , which is consistent 
 with partial reduction and two populations of pt oxidation states . 
 focusing on the pt 4f7/2 region , the peaks at 71.8 
  0.2 ev correspond to zerovalent dens . the black vertical line 
 at 71.3 ev marks the value previously reported for the 4f7/2 peak for pt dens .
in addition to these 
 zerovalent pt 4f peaks , a second set of 4f peaks , corresponding to 
 the g6-oh(pt)n precursor , 
 are also present .
these results are consistent with our previous finding 
 that the bh4 method leads to only partial 
 reduction of the precursor .
the 
 xps peaks in figure 4b were fit to quantify 
 the extent of reduction for each sample .
the fits are shown in supporting information figure s3 and quantitative 
 results are provided in table 1 .
in all cases , 
 the total xps spectra are well represented by deconvolution into peaks 
 corresponding to the pt and pt binding energies . 
 the extent of reduction determined from these fits is 59% , 40% , and 
 43% for g6-oh(ptn ) ( n = 55 , 140 , and 225 ) dens , respectively .
these values are somewhat 
 different than those we have reported previously for n = 55 , 147 , and 240 : 14% , 44% , and 64% , respectively .
we attribute the large difference in the percent 
 reduction of g6-oh(pt55 ) to the larger excess of bh4 used for reduction in the present set 
 of experiments ( 50-fold versus 10-fold ) .
figure 4c 
 shows xps spectra of the g6-oh(ptn ) dens 
 prepared by galvanic exchange . in 
 contrast to the spectra of the bh4-reduced 
 dens in figure 4b , these spectra exhibit just 
 one 4f7/2 peak corresponding to fully reduced pt .
these 
 results indicate that complete galvanic exchange has occurred with 
 the cu dens and that little or no unreduced pt salt or complex is 
 present in the sample .
xanes linear combination analysis ( lca ) 
 was used to further quantify the extent of pt reduction and to verify 
 the xps results .
figure 5a shows that the white 
 line intensity of the g6-oh(pt55 ) dens prepared by direct 
 bh4 reduction is between that of the 
 corresponding g6-oh(pt)55 precursor and g6-oh(pt55 ) dens synthesized by galvanic exchange .
the spectrum of 
 g6-oh(pt55 ) prepared by direct reduction and the fit obtained 
 from a linear combination of the g6-oh(pt)55 precursor and g6-oh(pt55 ) prepared by galvanic exchange 
 are shown in figure 5b .
the overlap is nearly 
 exact , and it indicates a percentage reduction of the precursor of 
 59% , which compares favorably with the value of 54% determined by 
 xps ( table 1 ) .
xanes data ( left ) and lca fits ( right ) 
 for ( a , b ) g6-oh(pt55 ) , ( c , d ) g6-oh(pt140 ) , and 
 ( e , f ) g6-oh(pt225 ) . for the right panel ,
the data corresponding 
 to the dens synthesized 
 by bh4 reduction are shown in black 
 and the lca best fit is in red .
xanes spectra for the n = 140 and 225 dens 
 and 
 den precursors are shown in figure 5c and e , 
 respectively .
the corresponding lca fitting for the g6-oh(pt140 ) and g6-oh(pt225 ) dens ( direct bh4 reduction ) in figure 5d and f , respectively , 
 provide good matches .
the resulting percentage reductions of the precursors 
 are within 7% of those determined by xps ( table 1 ) .
exafs spectra were acquired to obtain 
 the coordination number for the fully reduced g6-oh(ptn ) dens synthesized by galvanic exchange .
the fits yield coordination numbers of 8.9  1.2 for g6-oh(pt140 ) and 9.2  1.2 for g6-oh(pt225 ) .
these 
 values can be compared to those calculated for 140-atom and 225-atom 
 truncated octahedra of 9.09 and 9.49 , respectively .
these values are substantially higher than those reported 
 previously for pt dens reduced with bh4 : 3.99  0.61 for g6-oh(pt147 ) and 6.09  0.42 
 for g6-oh(pt240 ) .
the latter 
 very low coordination numbers indicate the presence of a substantial 
 number of unreduced pt species , which is consistent with the previously 
 discussed xanes and xps results .
an important outcome of the exafs 
 analysis is that in the future it will be possible to use in situ 
 electrochemical exafs to study electrocatalysis at fully reduced pt 
 dens .
exafs data ( black ) and r - space 
 fits ( red ) ( k - weight of 2 ) for pt dens prepared by 
 galvanic exchange .
the synthesis is carried out by first preparing cu dens 
 of the desired size , and then using the process of galvanic exchange 
 to convert these into pt dens containing the same number of atoms . 

we also re - examined the originally reported method for preparing pt 
 dens by direct reduction using bh4 , and confirmed that this approach leads to a 
 bimodal distribution of fully reduced dens and fully unreduced precursors .
the present research focus of our group is reconciling the experimentally 
 measured and theoretically calculated electrocatalytic properties 
 of dens . due to their high activity for many catalytic reactions , 

pt dens are among the most important materials for developing this 
 type of structure  activity relationship .
the types of future 
 studies we envision require , to the maximum extent possible , homogeneity 
 in catalyst size and structure , and the results reported here move 
 us a step closer to this goal .
 here 
 we outline a new method for synthesizing fully reduced pt 
 dendrimer - encapsulated nanoparticles ( dens ) . 
 this is achieved by first 
 synthesizing cu dens of the appropriate size through sequential dendrimer 
 loading and reduction steps , and then galvanically exchanging the 
 zerovalent cu dens for pt . 
 the properties of pt dens having an average 
 of 55 , 140 , and 225 atoms prepared by direct chemical reduction and 
 by galvanic exchange are compared . 
 data obtained by uv  vis 
 spectroscopy , x - ray absorption spectroscopy , x - ray photoelectron spectroscopy , 
 and high - resolution electron microscopy confirm only the presence 
 of fully reduced pt dens when synthesized by galvanic exchange , while 
 chemical reduction leads to a mixture of reduced dens and unreduced 
 precursor . 
 these results are significant because pt dens are good 
 models for developing a better understanding of the effects of finite 
 size on catalytic reactions . until now , however , the results of such 
 studies have been complicated by a heterogeneous mixture of pt catalysts . 
Introduction
Experimental Section
Results and Discussion
Summary and Conclusions