the area of nanoparticulate colloidal matter prepared from noble metals has diversified rapidly with the recognition of these materials as useful tools for optics , sensing , catalysis , release agents , and so on.1 this diversity is associated to their favorable surfacetovolume ratio as well as their related and interesting physicochemical properties.2 use of colloidal matter , providing large surfacetovolume ratios , exploits specific and desirable assets of the materials . these assets emerge from associated surface energies that vary with surface crystal orientation and facets , curvature , and topology . with the availability of colloidal gold , whose preparation was pioneered as early as the 1950s,3 its catalytic properties were soon recognized and used in technological applications . in the meanwhile , such colloidal gold nanoparticles are commercially available in a variety of shapes and sizes , as well as with associated surface topographies , for a range of applications from biomedical studies to analytical engineering implementations.1,4 
  the catalytic activity of colloidal gold has mainly been explored for electrontransferassociated reactions,5 among other catalytic uses.6 a desirable asset of colloidal nanoparticles is their good dispersibility in solution , if appropriately stabilized . in fact , immobilizing such particles on suitable supports provides an additional diversity in their microengineering applicability.7 benefits are easy removal from reaction solutions or flowthrough operations , reuse for catalytic implementations , and prevention from agglomeration of individual nanoparticle entities . we herein report the first preparation of suitable , robust , highly porous , and easytohandle monolithic hybrid supports,8 carrying a wellexplored gold nanoparticle load and catalytically active surface . these highly porous scaffolds enable liquid flow at low back pressure8c and demonstrate facile and robust reusability without any sign of clogging , together with excellent operational stability . we describe the preparation of hybrid organic  inorganic monolithic materials8b in situ in the confines of 100 m i.d . fused silica capillaries ( figure  1 ) . the pendant vinyl groups inherently existing in these multiporous scaffolds8 are then functionalized in situ with 1,3propanedithiol by means of thiol  ene addition followed by reduction of the disulfides by a dithiothreitol solution . this was achieved by simply flushing colloidal solutions of spherical gold nanoparticles through the hybrid monoliths until the effluent appeared to have the same color as that of the solution pumped into the capillaries ( figure  1 ) . fused silica capillaries carrying gold nanoparticles , as well as model catalytic reduction reactions ( 1 ) and ( 2 ) in an aqueous fluid phase containing sodium borohydride . figure  2 shows scanning electron microscopy ( sem ) images of a single globular feature ( example shown in figure  1 ) of the asprepared and washed pristine multiporous hybrid materials ( figure  2a ) as well as those of variants carrying gold nanoparticles with sizes of 5 ( catalyst 1 ) , 10 ( catalyst 2 ) , 50 ( catalyst 3 ) , and 100 nm ( catalyst 4 ) . in images b , c , d , and e , the existence of colloidal gold is clearly indicated . in order to assess the amount of gold bound to the hybrid scaffold , we washed catalysts 14 with freshly prepared aqua regia and measured the gold content by inductively coupled plasma mass spectrometry ( icpms ) ( for details , see supporting information ) . the results of this analysis ( table  1 ) clearly suggest that a larger number of gold nanoparticles ( recalculated from the mass of gold loaded on the scaffold ) of smaller size can be loaded on identical pristine hybrid polymers . this increased particle load ( over three orders of magnitude ) is enabled by an increased available surface for the gold nanoparticles to associate to thiol groups . smaller particles are likely to further ingress into the multiporous hybrid polymer structure with a significant population of mesopores that are smaller than 1020 nm ( figure s2 ) . this situation also explains why the number of loaded gold nanoparticles of 10 , 50 , and 100 nm size ( catalysts 24 , table  1 ) is significantly lower than that of nanoparticles of 5 nm size . interestingly , however , the total gold surface area estimated per unit of capillary length is similar for the 50 ( catalyst 3 ) and 100 nm ( catalyst 4 ) nanoparticles ( table  1 ) . sem images of the single globular features of the vinylposs hybrid polymers with an example capillaryscale crosssection shown in figure  1 for ( a ) the pristine hybrid polymer indicating an appreciable amount of nanopores , and ( b  e ) for goldnanoparticleimmobilized variants ( catalysts 14 ) clearly showing the internal structures with gold nanoparticles . determined mass , number of particles , and surface of gold per 13 cm length of 100 m i.d . samples were obtained by flushing the goldcontaining monoliths with aqua regia as explained in detail in the supporting information . based on the assumption that the determined mass of gold consists of particles of a given size and spherical shape ; calculated by using of 19.32 g  cm for the density of gold . calculated overall surface of the particles based on their number and size assuming spherical shape . this allows for a comparison of the performance of the prepared flowthrough catalysts with various sizes of bound gold nanoparticles at a closetoequivalent catalytically active surface area ( vide infra ) . it is well established in the literature that the catalytic activity of colloidal gold is dependent on the size of the gold nanoparticles.7 qualitatively , this was also observed in the present study with batch solution experiments of the studied reactions with diluted citratestabilized pristine gold nanoparticles of the same size as those immobilized on the solid monolithic hybrid supports . we performed experiments on identical gold surfaces with various gold nanoparticle concentrations , leaving other reaction conditions unchanged ( figure s4 ) . the results reaffirm that the performance of smaller gold nanoparticles ( 5 and 10 nm ) is clearly superior to that of larger gold nanoparticles ( 50 and 100 nm ) , while an overall trend of an improved apparent catalytic performance is found toward smaller particle sizes , a situation well in agreement with literature reports.7 estimated apparent rate constants from figure s4b are in the same range as those typically found in the literature,5b which allows us to gauge the performance of the flow reactors . when looking at the performance of our flow reactors ( catalysts 14 ) with a determined porosity of 82  % ( see supporting information ) , we consistently found high conversions of pnitrophenol and high yields of paminophenol at very low fluid contact times of a mere 1 min ( figure  3a ) . catalyst 2 has the best performance ( with near complete 100  % yield for pnitrophenol within a timescale of 714 s ) , followed by catalyst 1 , and then by catalysts 3 and 4 . importantly , the total amount of gold surface per unit volume of the capillary varies and spans more than one order of magnitude ( table  1 ) . this situation can simply be explained by a significantly different ingression of gold nanoparticles into the multiporous hybrid support with goldnanoparticlesizedependent surfacetocapillary volume ratio . ( a ) impact of fluid contact time in the reactors on the yield obtained for the catalytic reduction of pnitrophenol with catalyst 1 ( filled squares ) , catalyst 2 ( empty circles ) , catalyst 3 ( filled triangles ) , and catalyst 4 ( empty diamonds ) . ( b ) longterm stability of catalyst 2 in the steady state at a fluid contact time of 1 min and monitored over several reaction cycles with an overnight washing step and fresh reactant solutions provided after each cycle . other conditions : 0.3    10 mol  l 
 pnitrophenol in water containing sodium borohydride ( 30    10 mol  l ) . the reactions were performed at a temperature of t = 25 c . in longertimescale experiments ( example of catalyst 2 ) , we observed a decrease in the yield during continuous operation ( figure  3b ) . in order to test whether this is associated to a loss in catalytic activity or structural changes in the reactor , we flushed catalyst 2 after each cycle shown in figure  3b overnight with deionized water . this was followed by operation with fresh reactant solutions , and this led to the reestablishment of catalytic performance , indicating that the structure of the catalytic material remains unaffected for at least 14 cycles , one cycle being performed each day . we believe that the yield decrease is associated to the instability of the sodium borohydride in the nonbuffered reactant solution , which is continuously delivered by a loaded syringe over significant timescales.9 this instability also raises the ph of the nonbuffered solution over time . coming back to a discussion of catalytic performance and the size of the gold nanoparticles in solution and flow experiments , we find a similar relationship between gold nanoparticle size and catalytic performance ( expressed in mol  s based on the same fluid contact time ) for catalysts 14 , except that the best results were observed for a gold nanoparticle size of 10 nm , that is , catalyst 2 ( figure  4a ) . this may have a simple explanation because the larger content of gold nanoparticles emanates from their more pronounced ingression into the nanoporous structure of the hybrid polymer . increasing confinement by the hybrid polymer then leads to a possibly reduced active gold nanoparticle surface that is accessible to the reactants to undergo electrontransfer reactions . though the estimated surface area of colloidal gold is very similar for catalysts 3 and 4 ( table  1 ) , catalyst 3 clearly performs better ( figure  4a ) , an aspect that can be attributed to a size effect of the particles . ( a ) rate of reduction of pnitrophenol by catalysts 14 calculated for a fluid contact time of 0.42 min ( figure  3a ) , except that for catalyst 2 , which was calculated for a fluid contact time of 0.13 min , together with the theoretical gold surface area for catalysts 14 ( from table  1 ) versus nanoparticle size . ( b ) rate of reduction of pnitrophenol with catalysts 14 in flow normalized by the estimated gold surface area ( empty circles ) together with that for the batch solution experiments ( empty squares ) at a reaction time of 5.3 min versus nanoparticle size . the reactions were performed at a constant temperature of t = 25 c in both cases . to deepen this insight , we had a look at the goldsurfaceareanormalized rate of conversion ( expressed in mol  s  m ) per flow reactor , which is by far much worse than that in the solution experiment for catalyst 1 ( figure  4b ) , that is , roughly by an order of magnitude . also this value indicates that a significant amount of the estimated colloidal gold surface ( table  1 , figure  4a ) remains catalytically inactive for catalyst 1 , while catalyst 2 shows better performance than that in the solution experiment for this qualitative comparison . interestingly , the use of 50 and 100 nm gold nanoparticles leads to excellent performance again when the estimated surface area of gold is taken into account : the performance is at least comparable to that in the batch solution experiments ( figure  4b ) . such nanoparticles are too small to significantly ingress into the hybrid polymer structure and are primarily located on the external surface of the structural features and close to the micrometersized flowthrough pores ( e.g. figures  1 , 2d , and 2e ) . transport of reactants and products to and from the catalytically active gold surface is most efficient without the necessity to undergo hindered diffusion in pools of stagnant fluid . the observation described above demonstrates for the very first time that a careful tailoring of the nanoporous structure of the scaffold very much impacts the gold load and the overall apparent catalytic performance , and yet it can be clearly enhanced with respect to that of batch solution experiments when performance is estimated per gold surface area , an aspect clearly attributable to the continuous operation of the catalytic reactions . this is because fresh reactant is continuously supplied to the reactor and the resultant product is removed . we find that catalyst 2 shows the best combination of gold nanoparticle load , nanoparticle size , and catalytically active surface ( figures  3a and 4b ) . having shown that the materials presented here possess excellent catalytic activity in the typical reduction reaction of pnitrophenol , we also show their utility for the reduction of hexacyanoferrate(iii ) . since the reaction is rapid , we chose a ph value of 11.5 ( adjusted with sodium hydroxide ) , which drastically increases the halflife of sodium borohydride.9 figure  5 shows results for the pristine polymer ( used for thiol  ene addition and gold conjugation , figure  2a ) . we consistently found no reduction reaction on the pristine material at increased sodium borohydride concentrations varying from 0.4 to 80    10 mol  l , the latter being more than twice the equimolar amount of that of hexacyanoferrate(iii ) ( empty squares in figure  5 ) . on the goldcarrying scaffold catalyst 2 ( figure  2c ) , we found an increased conversion at increased sodium borohydride concentration under otherwise the same conditions as those for the pristine material ( filled squares in figure  5 ) . the conversion asymptotically approaches values between 90 and 100  % at an excess amount of sodium borohydride . this situation is consistent with the typical assumption made in the literature that the reaction rate constant becomes independent of sodium borohydride concentration at concentrations much larger than those of hexacyanoferrate(iii).1a furthermore , it elegantly shows the catalytic activity of the immobilized gold nanoparticles . impact of the sodium borohydride concentration on the yield obtained for the reduction of 40    10 mol  l hexacyanoferrate(iii ) at a ph = 11.5 ( adjusted with naoh ) and a constant fluid contact time of 1 min by the pristine hybrid polymer ( empty squares ) and by catalyst 2 ( filled squares ) . in summary , we have demonstrated the preparation of gold nanoparticle@polyhedral oligomeric silsesquioxane hybrid scaffolds , with a set of gold nanoparticles ranging from 5 to 100 nm in size , and elucidated their use as efficient platforms for green synthetic applications in solely aqueous fluid phases . our results show that these materials can be highly efficient catalytic platforms with robustness and reusability over many catalytic cycles in continuous flow operation ( figure  3b ) . the overarching best performance is achieved with gold nanoparticles of size 10 nm ( catalyst 2 ) . this is substantiated by the yield versus fluid contact time curves ( figure  3a ) as well as the related performance characteristics ( figure  4 ) . the results demonstrate that a careful combination of the porous structure of the scaffold and the chosen gold nanoparticle size drastically impacts the overall catalytic performance . our study further hints toward future endeavors for this type of reactors for catalytic screening applications involving colloidal matter as well as the integration of the prepared devices into more advanced microengineering platforms . in the present case , we currently explore the option to modify the pristine polymer with more hydrophilic dithiol linkers that will certainly lead to a better solvation of the material in an aqueous phase and , consequently , may lead to a potential impact on the catalytically active internal structure due to solvation and swelling effects . such an approach should be accompanied by an advanced characterization of the porous structure in the solvated state.10 related effects may influence the ingression of gold nanoparticles into the hybrid polymer structure and also the overall catalytically active gold surface area provided by these microfluidic device elements that allow for operation timescales in the order of seconds . porous hybrid monolithic scaffolds were prepared by a singlestep molding process involving covalent anchorage to the confining capillary . subsequently , the scaffolds were modified with pendant thiol groups , used as anchor for the gold nanoparticles , to prepare readytouse catalytic reactors operated in a continuous fashion for the two studied reactions . the analytical determination of the amount of gold within the reactors was performed by icpms after dissolution with aqua regia . further experimental details of scaffold preparation , modifications , and catalytic reactions as well as analytical studies on the materials and their results are provided in the supporting information .
abstractwe report on the preparation of new hybrid organic  inorganic multiporous monolithic capillary columns carrying gold nanoparticles of 5 , 10 , 50 , and 100 nm size and their use as flowthrough catalytic platforms for aqueous liquidphase reduction reactions . 
 we found that the flowthrough performance of the reactors depends not only on the size of the gold nanoparticles but also on the interplay of the pore size of the scaffolds and the catalytically available gold surface within the system , that is , loading an increased number of gold nanoparticles of smaller size does not necessarily result in strictly improved performance . 
 this indicates the importance of the interplay between the nanopore size of the scaffolds and the catalytically active gold surface existing within the system . 
 demonstration of the highly efficient catalytic flowthrough operation within seconds and the repeated use of the reactors without loss of performance indicates their excellent suitability as microfluidic device elements .